Definitely strive for the biggest tank you can get. 5 gallons is great for a betta. You can buy one secondhand (you WILL have to clean it well, haha) for very very cheap. If you have a Walmart, they sell 5 gallons AND *adjustable heaters. For $30. (It comes with other stuff, but the other stuff was deemed useless by me, lol. It includes conditioner, two plants, and food, although I would not use the food).
At the same walmart, they should have Tetra ____ Heaters, for about $12. Cheapest I can find, but they're preset. It does its job, yet not the best. If you have money to burn, i would go steps higher and get adjustable heaters. I want to say heaters are necessary, but will have to instead, say "they're very, very recommended." Bettas are tropical fish, so they naturally live in warm waters (77-80F). IMO 75 is pushing it. And the lower it is, the lower the immune systems of bettas will be, exposing them to being more sickly.
For the tank, you can buy at Craigslist - it seems to prove itself with its amounts of tanks I've seen for sale. And for pretty cheap too.
For the tank itself, I would recommend a fluorescent light bulb. They look fabulous with a tank. Incandescent, not so much. Plus incandescent will heat the water if on too long (even 6 hours, which is less than the average amount of light on. Or so this happened in my 5 gallons). Turning on, then off, will bring too much of a temp. fluctuation.
When I say adjustable filter, I mean just that... a filter with adjustable flow. Bettas cannot swim well against strong current, so minimize enough so that he's not fighting, but that the filter is rendered useless.
I also recommend buying aquarium salt, as it will help you a lot in case your betta gets sick. I prefer this, and CLEAN WATER (water changes) over medications.
You will need water conditioner, but it seems you do have a betta already, so I will assume you have conditioner. If not, if you just leave your water out for 24 hours, then I would highly recommend "Seachem Prime." Works faboo for me.
And when doing water changes, you might want to buy yourself a siphon/gravel cleaner/gravel vacuum (these are all the same, just different names).