I'm guessing that this is going to be your first gar, so your best bet is to get a Florida gar rather than a spotted gar since they're much easier to come by, are the cheapest gars to purchase, and look almost exactly like a spotted gar to someone who is not an expert on identifying gars (which is exactly why Florida gars are sometimes sold as "spotted gars" or "Florida spotted gars"). You'll need to have at least a 180 gallon aquarium in order to adequately house a Florida gar for life, but you should grow it out following the step-up system (gradually increase the aquarium size rather than place a small gar in a large aquarium). Every other species of gar will have at least the same space requirements, if not more. Alligator gars and longnose gars will definitely need a larger aquarium; Cuban gars and tropical gars might need a larger aquarium depending on their growth rate; spotted gars and shortnose gars have similar space requirements to the Florida gar.
As for acquiring one, I'd say that you should find one in no time just by posting a thread in the MFK Marketplace or by contacting certain vendors that regularly have them available.