No worries... just work with what you have... I had a baby betta female in a one gallon w/ heater for three months.
She had a handful of pebbles at the bottom of he tank, and one silk plant, and was perfectly happy. It was room for her, because she was tiny, and I did a 50% and 100% water changes each week.
When I got my sorority tank cycled, she moved into a spacious 20H, and now is the "leader of the pack".
So, work with what you have now, and move your betta when you can. I don't know if you have a bookshelf free, but here's a cool tank that is long but not big: