If you don't mind putting them up for sale in an auction, a number of fish clubs have them, especially in the spring and fall. If you live in a large metropolitan area these auctions are fairly common. Even regional centers in the countryside (population 20,000-100,000) often have terrific general clubs.
I've given a lot of guppies to various school teachers too. Before giving them any, ask how cold the school gets over weekends. (Too low a temperature in these money saving days and aquarium heaters can't keep up.)
When I had aquaria in one of my classrooms, I discovered not only that heaters were really needed (the 20 and 29 gallon tanks were more stable than the 10s) but because sometimes kids play with heater dials, submersible heaters (more expensive, but often more reliable) were necessary.
Another possibility is to put a low cost want ad in a local community paper or a freebie in one of those "shoppers".
If you belong to some sort of neighborhood association, local fraternal group or a religious community, sometimes their publications have room for a blurb like "free to a good home...." ;)
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