It isn't too hard to get a bigger tank for them. It doesn't have to be an acutual fish tank. A large glass bowl or any type will do. Just try to get something in the minimum of 2 gallon.s Most fish stores will sell 2.5 and 5 gallon tanks (just the bare tank, not a kit) for really cheap. Or else Eclipse makes some nice small all in one kits with light and filter.
People may say that is overkill for a betta, but once you see these guys swimming around a tank and brilliantly coloured, exploring plants and rocks, it is surely worth the price.