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Where can I find edible plants like duckweed, anachris, etc?
Petco and Petsmart don't carry them.

Try eBay... a lot of the sellers have a variety, though if you're looking for specific varieties, I would contact the seller to see what can be arranged.

See if you can find a tropical fish aquarium store in your area. Google "aquarium retail" + the name of a larger city near you and see what comes up.

The stores that specialize in fish are the ones who always carry plants, at least that's the case around here.