From Bulbapedia, the community-driven Pokémon encyclopedia.

A Poké Mart (Japanese: フレンドリィショップ Friendly Shop) is a convience store that sells supplies required for Pokémon training. They sell various items such as Poké Balls, Antidotes, Mail, and Escape Ropes. Their stock varies with their location. They also buy many different items in exchange for money. Much of the merchandise sold at such stores is produced by either Silph Co. or the Devon Corporation.

Almost all cities, both major and minor, have Poké Marts, the exception being some which have local medicine shops. Notably, the town a player starts in during each of the four generations does not have a Poké Mart.

An interesting thing to note is that in the anime, Poké Marts do not seem to be as ubiquitous as in the games.