For a betta? I just use those one-gallon glass fish bowls. But I can imagine a plastic tub would be cheaper. The fish bowls were gifts because someone heard I keep goldfish. And goldfish obviously go in bowls.
I go to garage sales and pick up 1 gallon containers I find for cheap. I have found them for like a dollar. and sometimes i get lucky and find ones with extra decor. Or you can just go to walmart and get a fish bowl for like 6 bucks.