There are a few true FW species (or rather, FW species that retain a similar appearance/behavior to flounder), but usually what you see sold is a Hogchoker, which does best in brackish and maxes at around 8". The other species tend to be rather expensive and only come in as special orders (at least around here).
These are a pain in the arse to keep if you buy them small, very difficult to feed. But when purchased at a larger size, much easier to care for. And very interesting fish that have been successfully kept long term in FW on many occasions to my knowledge, not personally recommending it per say though. I used to have one, really neat fish, but it was really fragile and easily injured (couldn't house it by itself in such a large tank and they can't be kept with small fish or inverts as they'll eat them, so I re-homed it). There's quite a few keepers over on MFK if you want more first hand experience/info with the species.
These are a pain in the arse to keep if you buy them small, very difficult to feed. But when purchased at a larger size, much easier to care for. And very interesting fish that have been successfully kept long term in FW on many occasions to my knowledge, not personally recommending it per say though. I used to have one, really neat fish, but it was really fragile and easily injured (couldn't house it by itself in such a large tank and they can't be kept with small fish or inverts as they'll eat them, so I re-homed it). There's quite a few keepers over on MFK if you want more first hand experience/info with the species.