Pokémon FireRed and LeafGreen/Route 24

These two routes contains a massive amount of trainers that you will have to battle in order to reach Bill's House. Both of these routes contains Grass piles and watery parts that provide a big variety of Pokémon.

Route 24
Route 24 starts at the bridge at the north part of Cerulean City. On this bridge you will have to fight five trainers (you can heal after each trainer and still receive your reward) and then a Team Rocket Grunt, who will give you a nugget before asking you to join Team Rocket. If you go west after the bridge there is a trainer, and piles of grass and water that contain this route's Pokémon.

Route 25
Route 25 starts a bit east to the end of the bridge on route 24. It's got even more trainers and at the end of it you can find Bill's house. When you get to Bill's house, you will see a. Talk to him and he will go inside the machine. Then, go the PC and press. He will come out from the other side as Bill. Talk to him and receive the S.S Ticket.

Items

 * Nugget (given by TR Grunt after you beat him)
 * TM45
 * TM43
 * S.S Ticket (given by Bill after you rescue him from the suit.
 * Pecha Berry (hidden west to the upper pile of grass)
 * Elixir (hidden above Hiker Franklin at the beginning of route 25).
 * Oran Berry (hidden at the end of the bushy route in a "bush corner")
 * Bluk Berry (hidden on the north east corner of route 25 before Bill's house)
 * Ether (hidden at the most east part of route 25 a couple of steps from the fence)