Walkthrough
After obtaining HM Flash from Professor Oak's Aide on Route 2, you can now proceed east of Cerulean City on Route 9. Route 9 will lead directly into Route 10, which is where Rock Tunnel can be found. Loads of Wild Pokemon and Pokemon Trainers can be found on Route 9.
Wild Pokemon
Grasses:
Rattata (40%): Levels 14-17
Spearow (35%): Levels 13-17
Ekans (25%): Levels 11-17
Pokemon Trainers
Picnicker Alicia: Level 18 Oddish, Level 18 Bellsprout, Level 18 Oddish, Level 18 Bellsprout
Hiker Jeremy: Level 20 Machop, Level 20 Onix
Camper Chris: Level 21 Growlithe, Level 21 Charmander
Bug Catcher Brent: Level 19 Beedrill, Level 19 Beedrill
Bug Catcher Conner: Level 20 Caterpie, Level 20 Weedle, Level 20 Venonat
Camper Drew: Level 19 Rattata, Level 19 Sandshrew, Level 19 Ekans, Level 19 Sandshrew
Hiker Alan: Level 21 Geodude, Level 21 Onix
Hiker Brice: Level 20 Geodude, Level 20 Machop, Level 20 Geodude
Picnicker Caitlin: Level 23 Meowth
Pokemon Trainers - VS Seeker Rematch
Picnicker Alicia: Level 25 Meowth
Picnicker Alicia: Level 30 Meowth
Picnicker Alicia: Level 55 Persian
Hiker Jeremy: Level 30 Machoke, Level 28 Onix
Camper Chris: Level 24 Growlithe, Level 24 Charmander
Camper Chris: Level 29 Growlithe, Level 29 Charmeleon
Camper Chris: Level 54 Arcanine, Level 54 Charmeleon
Items
Burn Heal: On Ground
TM44: On Ground
TM40: On Ground
Ether: ItemFinder
Chesto Berry: ItemFinder
Rare Candy: ItemFinder