I discovered that keeping mosses in too much light, may encourage hair algae to grow among them. Putting the mosses in a dramatically darkened place will eventually kill off the hair algae. So will putting a lot of heavy feeding plants in the aquarium, especially if they shade the moss.

Mosses "like" wet conditions. Make sure whatever you seek really is aquatic or can be. There used to be these little "Christmas trees" sold (a long time ago) in dime stores. It was some sort of clubmoss and it just sat there forlornly and died after a couple of months.

Here are a few places to look. I wouldn't ship if temperatures are not under the 80sF/ over 27C. By the middle of October it will be getting pretty cold to ship in the Northern Hemisphere or at least in the Great Plains.

http://www.aquariumgarden.com/index.php
terribly high price for JM

http://www.azgardens.com/

also Christmas moss, which has a different pattern and even greener. Additionally it is called Asian Christmas Moss/Taiwan Moss  (Vesicularia fasciculata)

http://www.aquabotanic.com/
They have it. Lots of info on this site and groupings of plants for sale. Currently it is on "sale".

See also:
http://www.westerleighaquarium.com/
http://www.freshwateraquariumplants.com/Merchant2/merchant.mvc?page=FAP/CTGY/SAP

Froogle Java Moss, Vesicularia, Vesicularia dubyana maybe even Bryophytes.

I would also look for the Java ferns, another set of Epiphytes which do very well in aquaria. There are several strains and cultivars in the hobby. Since there are several species of Microsorium pteropus in nature, there will be more too. :)

Also try image searches - sometimes one will get into a specialty club or find a commercial site that one missed before.

APD Aquarium Plant Digest mailing list is a long running plant list. Archives for that and other important mailing lists are at http://fins.actwin.com/search.cgi
Search for things like Java moss sources, plant venders, and aquarium plant venders

Sometimes other groups will offer plants that go with their fish. Killinuts love the low light plants. If you go to the AKA site www.aka.org and click on The Fish (left side of the page). Click on current F&EL or Fish and Egg listing. Plants are often included. (Members get the F&E archives.)

Also, a word about Fontinalis antipyretica, this is a widespread plant, found all over the Northern Hemisphere. At one time it was actually used to dress wounds (hence it's scientific name.) It has roots, the Java, Christmas and many other mosses don't have roots. I have seem (when I can't remember) Fontinalis for sale. A close look revealed that particular offering to just be Java moss, there were no roots! Fontinalis is really a cool water plant, Java moss is probably a better bet in aquarium and paludariums.

I once saw a poison arrow frog tank with living sphagnum moss. That is found in bogy, acidic places like the Okefenokee Swamp. Near me there is just the Volo Bog and everything there is protected. I'm sure that it is available commercially.  There are a couple of species probably found all over the US and world.

Raider, don't collect locally without bringing someone (and maybe a rope) with you.

I have also found Elodea canadensis  (Anacharis in the hobby) in reasonably clean local streams. It is a good as an oxygenator. It needs a lot more light than mosses.

And if you bring something home, DON'T just put it in an aquarium. Look for hitch hikers. Read
Cleansing New Plants in Immediate Help. In fact do that before collecting and plan accordingly. :)