you'd actually be surprised at your options here. most people unfamiliar with bettas will automatically assume that their fighting fish reputation prevents them from coexisting with other fish.
just to give you an idea, my two bettas live happily (in separate tanks albeit) with lamp eye tetras, zebra danios, neon tetras, otocinclus, african dwarf frogs, black mollies and albino corydoras.
generally speaking, bettas can peacefully live with most non-aggressive fish and fish lacking similar pectoral fins, i.e. no gouramis.
[edited to say that my two are males... i'm sure having only females opens up a few options.]