Pokémon Red and Blue/Route 3

'After you receive the Boulder Badge, other Pokémon trainers will see you as a worthy opponent. No fewer than eight trainers wait to challenge you along Route #3. You'll have a tough struggle getting through them all, but you'll emerge from Route #3 a more powerful (and richer) trainer than ever. Stock up on Poké Balls to capture some more Pokémon in the wild too.'

On the way to Mt. Moon
There are eight disgruntled Pokémon trainers between Pewter City and the entrance to Mt. Moon, but they're pretty easy to avoid (by staying out of their direct line of sight). Still you may need the EXP: Mt. Moon is tough.

At the Mt. Moon Pokémon Center, you can pick up a black market Magikarp for 500. As for wild Pokémon, Jigglypuffs are only decent in Red or Blue, but Yellow players shouldn't move on without a Sandshrew in tow.