You shouldn't need a filter unless you're cycling it - but I don't think that's appropriate for a tank of that size (2.5 gallons). You'll have to do complete water changes at least once a week, filtered or not.
As for the heater, I'm not sure what voltage exactly, but I do know that you can get either the Hydor Mini Heater at most, if not all, pet stores, or a Tetra heater that I believe is most often sold at Walmart. Both provide adequate heating in smaller tanks and should keep the temperature at ~78 degrees.
Other things you might be interested in are some plants or other tank decor as bettas do enjoy places to hide. I wouldn't advise live plants, though, because with the frequent water changes and constant uprooting the plants wouldn't do well. You can, however, buy silk plants as long as the colors don't bleed, and they don't contain any wires or fragrances.