Haha, that's an excellent question! I use to find it at garden centers that sell pond plants but lately all I've been finding is water hyacinth. Sometimes I see if for sale at pet stores too.
I'll back the others on this: RES get largely herbivorous as adults, so lots of greens is a good thing. And yes, Anacharis will grow like a weed (as some consider it to be) if you give it a bit of space and light.