The Grimoire of the Four Winds is only sold at the Black Market Auction House, which has been relocated from its old home in the Veiled Stair to the Ring of Blood in Nagrand. The approach to the Ring of Blood, located at roughly 53,14 if you have coordinates available in your game, is crawling with level 100 mobs. However, once you get to the Ring of Blood itself, all guards are neutral. Madame Goya is in a small building at the back of the Ring, and offers much the same items as usual -- raiding gear, transmog gear, and of course, the Grimoire of the Four Winds. While bidding starts at 1,000 gold, be aware that this is of course an auction house, and you might find yourself outbid.
Reports on Wowhead suggest that either the tooltip or the manual itself is bugged for the moment -- players that have successfully bought the tome and tried to use it receive a message that the item cannot be used until level 90. It's likely we'll see this hotfixed or addressed in the future -- after all, if you're level 90, you're going to be on Draenor and not really caring about flight at all anyway! But if you've been looking for the Grimoire, just head to Nagrand and see if you can pick one up.