Depends more on what type of gears that you have than it does on the size of the wheels. If the gear levers are the same type as on a bigger bike the 24 inch bike will shift just the same way.
A bicycle store should be able to sell one to you, or you can order one online. Single wheel 24 inch isn't hugely popular, so you might have to do a bit of searching.
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...