Walkthrough
After obtaining the S.S. Ticket from Bill on Route 25 and defeating Team Rocket in Cerulean City afterwards, you can proceed south on Route 5. This is a small route that contains on a few Wild Pokemon and the Pokemon Daycare. Route 5 leads to Saffron City, but, unfortunately, you won't have access to it at this time. Instead, you will need to take the Underground Path 5-6 to reach Route 6.
Nidoran Male (FireRed): Other Trainer Requests Nidoran Female
Nidoran Female (LeafGreen): Other Trainer Requests Nidoran Male
Wild Pokemon
Grasses:
Pidgey (40%): Levels 13-16
Meowth (35%): Levels 10-16
Oddish(FireRed)/Bellsprout(LeafGreen) (25%): Levels 13-16