Yep, you can get it at any pet store, it's not very expensive at all. Most grocery stores actually carry cheaper brands in their pet aisle (not necessarily ones I would recommend purchasing, though), so you should have no trouble finding it. Seachem Prime is the best value for the money, Amquel+ is comparable, but not as concentrated so the bottle won't last you as long. You should really get this stuff sooner rather than later.
As for how you're keeping him, sure, there's more than one way to skin a cat. Some ways are easier than others. :) If you look at this from a long term perspective, with proper care your betta may live to be as old as 7 years old. The typical lifespan is 3-5, but it's certainly not unheard of for bettas to live well beyond that. Consider how much all these water changes are going to affect your lifestyle for the next 3-5-7 years--doesn't it make sense to invest a bit more money into your setup if it means drastically cutting back on maintenance?
If you upgrade later to a tank that's 3G or larger (the bigger the better and the further you'll stretch your dollar) with a filter and some live plants, you're looking at one partial change a week versus frequent 100% changes. From that point alone it's totally worth it. Also consider that you'll have a much greater error margin in case an emergency arises and you can't do your water change on time--god forbid your betta gets sick, and then you have to drop money on expensive medications, too.
A bigger, filtered tank doesn't have to be expensive either. You can get a 3G critter keeper for a few dollars and add a $4 sponge filter and a $6 air pump and you're good. A proper 25 watt adjustable heater will end up running you about $15. I highly recommend ordering online--trust me, pet stores jack up prices to the point that I don't even buy cat food in a store anymore. I purchase all my fish supplies here: http://www.fosterandsmithaquatics.com/
the product variety is great, flat rate shipping is totally reasonable, and the customer service is very quick to correct any mistakes. Sometimes they'll send you little freebies with your packages, too. ;x
Excuse the fact that I missed that you'd had the fish for two weeks
--somehow I came away from reading that post thinking you had just gotten the fish. I should really try to skim less, lol. Based on what you're saying there is a definite possibility that the fish has internal parasites. A distended belly and loss of appetite are both symptoms. Have you seen him pass any feces during this time? Can you describe it? Luckily, if this does happen to be the problem, fish respond extremely well to treatment and the medication seems to be quite easy on their systems. If you decide to treat him, I recommend a dissolving tablet medication with the ingredients praziquantel and metronidazole--I have had some success with Jungle's Parasite Clear tablets.