Bettas are primarily solitary fish, and most of them don't want nor need 'tank buddies.' If you must have something else in the tank besides the betta though, cory cats are a popular choice, as a school of them will help liven up a tank, and most (read: most, not all) bettas will leave them alone. Some people also go with snails (although some bettas will eat a snail given the opportunity) and some species tend to reproduce like mad given the opportuniry, so do your research on what kind you want before running out to the store for some.
Current Quarantine Roll-Call:
Copper-lavender DT found in a tank with an adult VT male, apparently they thought because of his size, he was female and it was okay to house them together? Badly shredded fins.
Don't you just love what you can find in local stores?