Aww, pretty boys! You came to the right place :)
I think a divided 5 gallon or divided 10 gallon would be perfect for you. Both would be fine for the bettas, but you'll probably have an easier time finding a 10 gallon. I have my current boy in a 5.5, and I had a heck of a time finding a replacement hood for it, so I'm inclined to steer you in the direction of a 10 gallon if only because they are so common it's easier to find parts. Larger tanks are also easier to keep clean, but, if you are worried about space, a divided 5 gal would be just fine for them.
5 gallons and 10 gallon kits cost about the same. You should be able to get a 10 gallon tank, with light and filter for around $30-$35 at Petsmart or Petco.
You will also need a heater-- bettas are tropical fish and one of the most commonly overlooked aspects of their care is heat. You will see a huge difference in the healthiness and activity levels of the fish if you can keep their tank temp at about 78F-80F vs room temp. Expect a 50watt heater to run you another $20 to $30. I know that's almost as much as the tank, but it really is important.
I do most of my shopping at Petco/Petsmart. You can often get better deals online, but I'm too impatient for shipping most of the time.
There are a couple of ways to divide a tank. I've only done it once, so you might wait for input from others-- you can buy dividers or make your own. With a divided tank, make sure you have a few plants (real or fake) near the dividers so the boys don't constantly see each other. Male bettas will feel stressed if they are in constant view of another male betta. It's also a good idea to make sure they have a hidey hole/shady spot. A clean ceramic coffee mug works just fine for this, by the way.
The good news is that once you get everything set up they really aren't difficult to maintain. I have 6 fish tanks going at the moment (*gulp*) and I spend about 2hrs a week doing maintenance. One divided fish tank won't be that much trouble once you get it set-up.