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 Pokémon trainers between Pewter City and the entrance to Mt. Moon, but they're pretty easy to avoid. Some of them can pose a threat since they are close to the same level as your Pokémon. Still, you may need the EXP.

Mt. Moon is tough because you will run into many wild Pokémon, as well as trainers who use a mix of a few Pokémon types. This is a good place to get a Pidgey or Spearow for your Flying-type, having a 90% chance (45% chance each) of getting one of them in Red and Blue and a 55% chance of getting a Spearow in Yellow.

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