No idea where you'd buy but after your betta you only have remaining gallon space for 3 gallons of water in each section, assuming you only use 1 gal per inch of betta, and that's not really room for anything other than maybe 1 fish. Most of your options then for a fish that small would be like a tetra, molly, etc, which would not be good to keep singly with a betta because one will pick on or nip the other. Those other types of fish are also schooling fish and would be stressed in that situation, so unless you do some rearranging, I think one betta with a snail, moss ball, etc would be perfect.
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