Petco usually does. Once in a while you can find some at Petsmart. Sometimes even walmart has them. But your best bet is probably to just tell folks on here you want to buy one. You may have to clean it up a bit, but you'll get it from someone who knows its history, knows its condition and will hook you up big time.
personally I like buying the kits. If for no other reason you generally end up with some essentials and some spare parts (like a heater if the one you have takes a sudden dive)
You just missed PetCo's $1 per gallon sale on tanks. It's a great deal and sometimes they even include the kits.
Also Craigslist is always a decent option too.
