There are several freshwater puffer species. The dwarf puffer is the probably the easiest to find, cheap, I think they recommend about 5g/puffer but you might be able to stretch that.
The next step up from that would probably be one of the red eye puffers (2-3" size)..carinotetraodon irrubesco being your best bet for anything community wise, although any puffer is pretty risky. Also the arrowhead or mekong puffer, they get about 5-6", freshwater, but definitely best in a species setup.
PufferPedia is a good resource, especially if you have access to rarer species:
http://www.**************.com/forum/ug.php/v/PufferPedia/Freshwater/
Through my wholesalers, the only freshwater puffers I've ever seen have been the dwarfs, red eyes, and mekong puffers.