Bluefins often turn up in dealers' plant tanks - I gather from Florida suppliers. They attach their eggs to plants, which subsequently hatch out in the dealers' tanks. I have also seen them (and other species, such as swamp darters) mixed among live foods - such as ghost shrimps. Just keep an eye out for them in plant tanks and among feeder shrimp shipments and you should be able to turn up some more. Otherwise, you could check on-line for native fish suppliers. They are pretty undemanding to keep, though will show much better colour (and will breed far more readily) if kept in an outdoor pond. The ones I kept in Phoenix, Arizona, survived throughout the year outside - so handled light frost as well as very nasty heat.
Tony