www.aquariumstuffers.com had them quite regularily but their site has been down for like the past 2 weeks so I don't know what is up with them. I loved aquariumstuffers because they have all the rare/hard to find brackish fish like Anableps, Mudskippers, etc. Anyway, good luck finding them. I have heard (this was several years ago-we tried getting some in and they refused to eat and all died) that they are actually a saltwater fish. Not too sure how accurate this is now however but I would guess they are a high brackish fish. Anyway, hope this helps and good luck!
I think these things are tank-busters, i.e. get quite large. Plus they are mouths with a tail, so not great companion fish. But I've never owned one, so. . .
It'll be by itself once the crawfish moves out of the tank. They aren't tankbusters and grow under 9 inches in most circumstances. The record was 12 inches.