Generally speaking, LFS (local fish stores) keep them individually in fist sized containers. Though it may seem inhumane, it's all about maximizing "sellable sq footage". Beleive it or not, they are shipped in from the Far East in sealed tea bag sized plastic bags with 2-3tbs of water and can live up to a month in there. Accidentally left a box in the corner back in the day when I imported/exported fish....whoops
Look for ones that are active , with a bit of "piss and vinegar" in them as they display aggression to bettas in adjacent containers and that the water is clean. The better LFS change the water every few days.
Food to feed them I recommend the Hikari Betta BioGold with freeze dried blood worms as a treat. No need to use frozen as one can get 3-4 treat sized portion from a frozen cube (~24-50 cubes in a tray depending on manufacturer)...unless you want to spoil them rotten like my wife does
when we had them.
Typically they live for 2-3years but one of my wifes' betta lived to an uncanny 8 years but the last 3 were spent lying on its' side on the bottom as it's swim bladder deflated due to age. Kept the water level @3" so it didn't have far to swim for food and MacGuyvered an auto top off system so the water level didn't get too low...the things I do for the wife :P