CRB p. 299: "New spells, rituals, or alchemical preparations (p. 316) can be learned by studying spell formulae or finding some mentor (either a spirit or another Awakened individual) to teach you."
My GM decided that since Chaos was not an well organized tradition, and that two chaos mages might not even recognize each other, that formula were difficult to find. He did allow me to learn spells from spirits, paying the "instructor fees" to the spirit in reagents. So I just pay price for low level instructor to match the formula cost, get a couple extra dice and learn my spell. Works well enough and makes sense to him fluff wise. He does not make it difficult to get reagents, though as a follower of rat, I can use any.