Potion

Heals 20 HP of a Pokemon. Has no effect on a fainted pokemon. There is one in your PC at the start of the game, except in Generation IV. It is 300 Poke dollars at Poke Marts.

Other locations
R/B/Y/FR/LG: Poke Mart worker on Route 1.

G/S/C: Poke Ball on Route 29.

R/S/E: Poke Mart worker in Odale Town

D/P/Pt: Poke Mart worker on Route 201