Where do you find Gyaradosite in Pokemon Omega Ruby/Alpha Sapphire?
If you get onto Route 123 from the Mt. Pyre side and jump along the top ledges, you'll find a shack called 123 Go Fish. Inside is a Poochyena. Talk to it and i't'll ask if you want to scratch it. Say yes and you'll get the Gyaradosite.
If you're still having trouble here is a video which shows you how to obtain it. Source
