Calypso, coming from someone who lives in a small flat, I understand that the suggestions of 10-gallon tanks for one fish are getting on your nerves. Personally, though, I keep my betta in a heated and filtered 2-gallon Fluval Spec now, and I've never seen him so happy and healthy. 1 or 2 gallon tanks really don't take up much space, many are designed beautifully to match your interior decor, and your fish will thank you for it. Plus, you won't have to change the water so often as in a 1/2 gallon tank.
Back to the original topic, if you must go to a chain pet store to buy your new betta, get up close and personal with them and watch their reactions. If they are active and respond to you, then it's a good, healthy one. Also check for any "fuzzies", white dots (looks like salt has been sprinkled on the fish) or strange discolouration. All of the above are signs of disease - avoid that like the plague, no pun intended. If you have a pet store specialising in fish in your area, or a breeder, then I'd highly recommend buying your betta there instead.