I use jars... I wouldn't use terracotta pots to be honest, if anything I would prefer the jars. Hunt around... Look in thrift stores, kijiji, etc, look for MEDIUM canning jars. That is what I have... Everything from medium to very large... I don't understand what you mean by "they'll ram the glass" no they won't. I have about 30 jarred currently, and have never had a problem. You could do the fish bowls, but they are more expensive than the mason jars... Go the cheaper route. If you can even, find cups with lids... You can also use clear Dixie cups and fashion lids to avoid jumpers.
OR if you have the tank space, buy a few "Betta Barracks" as they are called. That way cleaning is easier, they are all heated, and takes up less space.