Since you are on a budget, you probably don’t want to look at acrylic tanks. While they are lighter and stronger than glass tanks, they scratch easily and more importantly are much more expensive.

Also, while I want to respect your privacy, in a general sense where do you live? For instance I live in the southern ‘burgs of Chicago, in the Midwestern US.

Sales at shops do occur from time to time. It might not even hurt to ask a place you do business with as to when they might have their next tank sale. Summers are the worst times for shops because people are outdoors spending their money on other recreations and letting their fishy interests slide. Holiday sales might also be useful.

While you are in the shop you can encourage people to NEVER buy pets for Christmas, Easter or birthdays. ;)

Aside from the auctions, there are on-line places that will sell tanks. Glass boxes or something like that is one.  Sorry I don't have their URL. It may have been already mentioned on Guppylog.

Ah! Here it is:
http://www.glasscages.com/?sAction=ViewCat&lCatID=2

Also take a look at http://www.all-glass.com/
just to see all of the types of aquariums which are commercially available. And those are the offerings of just one place.

These will cost more than you wish to spend, but for examples of more elaborate systems, take a look at the UK’s http://www.juwel-aquarium.de/en/aquarium.htm

On the cheap side, there are a number of options for used aquariums. They range from very good condition to just cruddy with minerals. A lot of this is timing and a little opportunistic. The list I'm sure could be expanded.

For used equipment, very serviceable stuff will be often offered at about half retail price. If it is not a popularly use item, 25% retail might be more common.

I know that there are people who want full retail prices for their use stuff. If they know computers, ask them what they would pay for a four or five-year-old computer. Not everyone will get the drift, but hopefully they will. :)

Look in those little advertising newspapers tossed on your lawn or delivered in the mail. They will also sometimes be at the door of restaurants. They charge nothing or little for ads (unlike the big newspapers) and that is reflecting in asking prices. Often it is a tank a family just wants to get rid of.

With both ads and garage/yard sales, you might ask why they got out of keeping fish. If there is any indication that the fish died off mysteriously, I would suggest that you boil the gravel, if possible in porcelain container. Also bleach the tank and hard stuff. If there are other items like sponge filter sponges which you don’t trust to boiling or bleach, make up a super saline solution (of maybe 20 oz to a gallon) and soak those items in there.

Somehow I got a call from someone when we lived in a townhouse. It does pay to be known locally as a fish head. Down the way someone (perhaps as a result of a marriage breaking up?) was moving out fast. They had a couple of two-tiered rows of 2x4 stands with various marine aquariums on them. Unfortunately it was just before the school year and I was broke, We bought a little. If I had had much money I would have brought home several 20s with anemones still in them!

Since then we have bought stuff from friends who found it wasn’t cost efficient to move larger tanks across the country. As one’s “fishy reputation” has gotten out, others have asked us to baby-sit a tank, with the understanding that if they weren’t back for it by the end of the year, they weren’t coming back for it. ;)

Versions of the garage sale are the moving sales and estate sales. The items there may be a little better quality and a little more “pricey”. Interestingly, I don’t mind looking for aquarium gear at an estate sale, but feel really funny looking at other things which might be family treasures there.

Sometimes you will find a pet shop refitting or upgrading their fish display section. I picked up a bunch of 1-gallon tanks for half the regular price of 10s. More significantly, they must have been some “commercial grade” of tank, because the glass is twice as thick as in ordinary 10-gallon tanks.

Sadly, small and even big box shops are always going out of business. A lot of gear will be cleaned up and sold as a discount. Good

Fish clubs and auctions are also an excellent way of gathering equipment. This is especially true if you have the kidneys and perseverance to wait until the end. By then many people will have left for other responsibilities or because they have more than enough fish and must put something in a tank right away. Others will stick around, but have spent all the money they had planned on. Still others simply will not bid on most at the end of an auction because they already have what is being offered.

If you go to Goggle type in your city, region, province, state or country and Fish club or aquarium society you will get many of the groups within comfortable driving distance of your home. Personal contacts through fish clubs, should you find a club you are comfortable in, will also, in time, point you in useful directions. Many people in fish clubs are there for the fellowship, info and access to fish just not found elsewhere, but they don’t mind the extra can of food from the raffle or door prize drawing. Pure – or is that impure - 100% “takers” are not usually appreciated though. ;)

Someone building a fish room may have stuff they need to get out of the way. The nice thing about that is that the tanks are probably clean and more likely to be disease free.

Older aquarists will scale down their collection of tanks. I got an e-mail from a Michigan club I'm nominally affiliated with. A gentleman who must be in his late 70s or 80s finally decided to get rid of some of his tank. If I knew anyone locally who needed tanks, I would have made the run just to say hi to him.

I’ve kidded before about looking for aquariums, while driving to work, on garbage day. When we bought a house in the burgs (ok, made a deal with the bank to pay them a lot of money so I would have the privilege of mowing lawns, painting walls and falling off of ladders) we were aghast at the number of people, sometimes in pretty nice pickup trucks, who came around scavenging stuff. In time we can to see that as useful to people in many ways. Before recycling we came to leave newspapers out by a certain time for the guy who took them. And old lawn mowers, furniture, a pretty good sink (but I scratched it cleaning tanks), computer stuff and other items which might still be of use were put out the day before so the recyclers would get them before the garbageman.

I became a convert to the cause the day I saw a 29-gallon tank and 5-gallon out by the curb. That was the fastest not-so-legal U-turn I’ve ever made. The big tank had a seam leak, but it was fixed and set up a couple grad students with two nice aquariums.

Many of those curbside aquariums come out of basements or garages in the spring. A number of them were Christmas aquariums which failed, usually because they recipients knew nothing of cycling and aquarium or because an uncleaned under gravel filter eventually backed up as a biological time bomb and suffocated everything in the tank.

I knew a guy who was between jobs and had the time to do a little cruising on garbage days. That spring, he cobbled together a 15 or 20 tank fish room mostly with stuff he picked up!

Resale shops and rummage sales are fund raisers, which are also for worthy causes. One which we drop extra good but not likely to be used items at is run by a local humane society. Not a bad cause!  :)

For a time we took a guy in and very reasonably rented him a room. Close to broke as usual, I needed a car tune-up and offered him a ten-gallon tank with most of the trimmings in trade. Because of our frugal approach to gathering equipment, it was a good deal all around. So there are always ways to use “the underground economy” for gear.

On an impulse once, I called up a friend who once kept killies and asked him if he would like to ride along to a meeting of the Chicago Killifish Association. He said yes and away we went. During the mini-auction in the last portion of the festivities, where pairs of killies, plants, food cultures and the like usually go for $1 to $10  he was in there in there until the successful end bidding on some really rare and $38! I was delighted that he was back in the hobby. And then he leaned over and asked, “Um, do you have an extra ten-gallon?” So I suppose we can include gullible friends with more aquarium gear than is good for them.