You can find the Chocobo Sage’s House in the Icicle Area on the northern continent. There is no way to access it by foot, and you can only reach it after you receive the Highwind on Disc 2 while escaping from Junon.

You will have to visit the Chocobo Sage’s House numerous times during the completion of the Chocobo Racing and Breeding side quest, but the first thing that you should do as soon as you have access to the Highwind is travel to the Chocobo Sage’s House and obtain the Enemy Skill Materia by speaking with the green Chocobo on the bottom floor.

The Chocobo Sage sells the following items:

• Porov Nut
• Pram Nut
• Sylkis Greens
• Reagan Greens

He also provides you with a wealth of useful advice on how to raise different-colored Chocobos. However, you won’t need this because you can use the previously mentioned Chocobo Racing and Breeding side quest guide on this site to assist you.