Hello & welcome!
I have 2 bettas also. One in a community, and one in it's own tank.
My community has a heater, my little betta loves it!
My smaller tank stays in the living room, and it stays room temp, so I don't need a heater.
Also, I would recommend getting a few little plants for your bettas. Both of my tanks are planted and my bettas love it! They swim all through the stems of the plants and my male betta even lays on the leaves of the plants
They have such personality!
Also, about getting a bigger tank, I'm not sure if they have a Pet Supply Plus where you live, or if they have the same sale. But where I live they were having a sale that was 1 dollar per gallon. No matter what size. Anyways, good luck with your bettas!