My petco gets some of the most gorgeous bettas in...I have no idea who's supplying them, but they're lovely. But, I agree with the others, make sure you try and go in the day of the shipment or the day after, because the less time they spend in those cups, the healthier they'll be.
Right now, through April 27th, you can get a 20 gallon long aquarium for 22$ at petco. They're on sale. As for the hood and the light, you can do a DIY for about 15$ (and you'll have plenty of plexi left over for other tanks as well.)
All you need to do is go to a local hardware store and get some acrylic/plexiglass and have it cut to the size of your tank (leaving a bit of an overhang along the sides so it fits over the top) and leave a section along one side *short* so that you have room for your filter, heater, and lamp.
Then you can purchase a clip-on desk lamp from walmart (I got several for $7 recently), and you're good to go. :) Here's a picture of two of my tanks set up this way:
(excuse the mess, these tanks are in my catch-all/storage room...so there's always left-over furniture, clothes, things of that sort hanging around in there. :)
Since I did several tanks and was able to use all the acrylic that I bought, it cost me 10$ per tank for the lid and the lamp, so this is definitely cheaper than buying a hood. And I like the sleekness of a clear top on a tank, and being able to see my guys from above. :) (But that's just personal preference. :)
Oh, and one more, here's a link from amazon for the lamps if you can't find them locally:
(there's one about halfway down for $9, but there are several styles depending on what you like best. :)