Why dont you go with a bigger tank if you want natives? a 20 is simply a tiny tank for almost any native.
To answer the origional question, I got my bass and a sunfish from a nearby river as 3" babies. Had them in a 30 long which lasted less than a month before I had to thin the population and upgrade. The catfish I bought at an lfs.
Yes, a 20 gallon is small for the majority of gamefish, but natives as a whole, it's a good sized tank. There are hundreds of species that could work in a 20 gallon community tank with small minnows and darters. There are several beautiful minnows and darters, as well as the few sunfish species I mentioned before.
check out the native fish section of aquabid.com, they've got a good amount of smaller natives for sale. in a 20g i'd go for not more than 2 blue-spotted sunfish or black banded, some darters and some smaller minnows or killies
