my wife wanted one so we got a baby biorb for it (its 4 gallons) nice little size for a betta i think. although ours doesn't have one in there yet, it's still cycling.
depends what you want your tank to look like really, some people hate the biorbs, others like them (either way the filter isn't exactly the best, so you can't really overstock them) or maybe go for a fluval edge, or get one of the (bigger) betta tanks, there was one in a place near me which was about 3G, maybe 2.5, with an overflow at the back leading to the built in filter, even had a little space for a small heater back there, but didn't include one, and used LEDs on an arm for lighting. nice little all in one system, and theres other options, or you could go for a cheap tank, and get all the bits yourself... theres small tanks (but big enough for a betta) at around £15 near me, plus a small filter and heater and its about £40 al together...