Most stores and many breeders will keep bettas in tiny cups or jars. This is to fit as many bettas as possible in a small space as keeping them each in their own large tank would not be financially feasable for an industry trying to make money. Pet stores suffer massive death losses with bettas (in part due to too small of containers for the amount of waste they produce, I'm sure, not to mention the inadequate temperature), so if they bought even a gallon tank for each then lost half of them, they'd be losing huge amounts of money.
However, when you have your own betta, I wouldn't reccomend keeping it in a jar if you want it to live more than a few weeks. 1 gallon is considered the absolute minimum by most Americans who know something about the species, though with the tiny containers they sell in the stores, you should realize that few even get the priveledge of one gallon. I personally have found that bettas will live longer, healthier, and happier lives in larger tanks, 5 gallons being about minimum as this is the smallest tank size that is easy to keep heated, filtered, and "decorated" with plants and rocks for the fish's benefit. I consider 1 gallon and 2.5 gallons to be adequate when you first get the fish to slowly accustom it to larger tanks and filters, especially if the fish you get is sick (I happen to buy sick bettas soley for the purpose of rehabilitating them, and ones with fin rot and gill fungus can't handle big tanks and strong currents.). However, once the fish is healthy and strong enough for a mild filter and more space, I think that larger is better. My apartment only allows 5 gals, or mine would all be in 10's.
Now, you can technically keep bettas in cups, if this is what your asking, but consider the benfits of a good sized tank with a filter: Cleaner, freshed, better oxygenated water. Less concentrated contaminants. Weekly 25% water changes instead of daily 100% water changes. Room for a heater so you can keep the water temperature at the right level, and the temperature will remain more stable in larger amounts of water. While it may be more expensive than a cup, you're bettas will live longer so you'll buy fewer, and chances are they'll need less meds for ailments, less special foods for lagging apetites, etc. so in the long run it is better for you and the fish.
I hope this answered your questions ^^ I would read up on the betta forum, they have lots of great advice on betta care.
