Where do you get the rainbow pass on firered?

You cannot get a Rainbow Pass in Pokemon FireRed. The Rainbow Pass is an item exclusive to Pokemon Ruby and Sapphire. It is used to unlock the ability to enter the Battle Frontier.

There are no ways to transfer it from Ruby/Sapphire to FireRed, as there is no way to transfer items between generations.

It's important to note that while FireRed and LeafGreen are technically in the same generation as Ruby and Sapphire, the games were developed and released separately, and their mechanics and available items are different.