Beta's for the most part can be kept in pretty small tanks, they don't need filteration or air pumped in to the tank because they can breath air from the surface unlike most fish, they have special tanks at any local store, wal-mart, petsmart, ect.

Betas shouldn't be kept in a tank together unless there is a divider.
If you decide to get the divider, i would get a male and female or two females, because males could become over stressed.
This is a link showing this kind of tank, and the other is just standard beta tanks




http://www.petsmart.com/global/produ...N=2037042&Ne=2


http://www.petsmart.com/global/searc...N=2037042&Ne=2