I found a place almost at the very beginning to trainer-fly glitch in Yellow.
This trainer is only in Yellow, and sees you from the maximum distance. He is second last person in Viridian Forest, before the last U-bend:
How to use glitch:
1) Play until you reach Pewter. Get Escape Ropes and Pokeballs.
2) Use Pokecenter.
3) Go back to Viridian Forest and use trainer-fly glitch with Escape Rope.
4) Let the guy outside Pewter take you near Brock's gym.
5) Go and trigger a battle with a Pokemon with a special of 7.
6) Use Pikachu's Growl 6 times on that Pokemon.
7) Go back to Viridian Forest. Clear the message "Hi, do you have a Pikachu?".
8) Get L1 Nidoking.
9) Glitch L1 Nidoking to go to L100.
P.S. I keep getting "You missed the Pokemon" when trying to catch Nidoking.
P.P.S. Not anymore, at least if I damage it with a Pidgey.