Not sure of where you live, but a 10 gallon tank costs about $8.50 at most pet supply stores, keeps a stable temp easier than a smaller tank, and can be divided 3 or 4 ways with still relatively content bettas. But, if you want something smaller, you can buy the little 1 gallon containers. Though I must say, even the little triangular ones with the tiny light and filter cost over 10$. Not much of a deal if you ask me...
How many bettas are you keeping/what space do you have? That might be useful info for us to help.