Male bettas can get along peacefully in a tank with other non-aggressive, non fin-nippers, and non-brightly colored fish that could be mistaken as another male betta.

Goldfish, mollies, tetras, platys (I think?), guppies and more are all more than compatible with being in a tank with a male betta

We have a fish compatibility chart at work and I can grab one tomorrow and give you a list of all compatible fish that Petco carries if you'd like!

EDIT: flatcoatluvr, its funny you mention space because its not space that's an issue when it comes to male betta aggressiveness. I watched a special where they did a test. They went to this large lake somewhere in Japan I think and had two male bettas they released into seperate ends of the lake. Within minutes they found each other and attacked one another.