Where do you get the Legendary Pokémon in Pokémon Red?

Arcticuno: on the Seafoam Islands

Moltres: in the Victory Road

Zapdos: in the Power Plant

Mewtwo: after you beat Elite Four go to the Cerulean Cave, before the Elite Four was beaten, it will be blocked by some guy.

Mew: Look in the cheats.ign.com for the Mew cheat, it should be there.