It's not in the game.
It's not in this game.
berry forest
vermilion forest
Viridian Forest or the Power Plant.
Ilex Forest in not in Firered. Ilex Forest is located in Johto. It can only be reached in Gen II or in the unreleased Heartgold and SOulsilver in Gen. IV.
It is not possible to visit Ilex Forest in Pokemon FireRed as it's a location exclusive to Pokemon Gold, Silver and Crystal. Pokemon Gold, Silver and Crystal are the only Pokemon games that let you visit more than one Pokemon Region.
They are found in Viridian Forest, but are rare.
Search around in the berry forest.
With a FireRed game in the GBA slot (if your DS is an original or Lite design), you can find Caterpie in the Eterna Forest.
In Pewter City, past the Varidian Forest.
You can catch Pikachu in Viridian Forest and at the Power Plant.
go through viridian forest