I guess that kinda depends on where you're from. If you're mainland USA, most chain stores have great deals on 1-3 gallon, squat containers that bettas would love. If you're somewhere yonder USA... you'll have to wait for someone else to reply.
One container which I've personally been interested in, is the "Herp Haven Breeder", which is a long, squat tank.
You can order online from www.petco.com or www.petsmart.com, as well.
Another option, which I tend to favor, is the Salvation Army or Goodwill, or any garage sale. If you're antsy, you can go to a wholesale store and purchase a big old container of pickles or pepperocini for around $3, enjoy several very good sandwiches, and have a perfectly serviceable container. A deli is another place you can get these containers, usually for free. The big ones are just shy of a US gallon.
However, if you want something intended for fish... uhn... any place with a big chain-store name, will generally have better prices on plastic tanks, than a small mom-and-pop store.
On the other hand, if you find a good mom-and-pop store, cling to it like a life line. It'll be really rewarding.