Where can you buy fixie bikes?

Don't think you can, unless you pay someone to build you a 24" wheel round a fixie hub. Fixed gear bikes are almost only used for road/track cycling, while 24" is MTB DH/ Dirt territory, where a fixie would be really awkward.

Easiest kludge is probably to find a bike with 24" disc brake wheels, then slap

on a Tomicog. I'll post a link about them. Remember that you need to ditch the rear derailer, and that the frame has to have horizontal dropouts.