Every spec has 1 model variation (the 6th in the calculators) which are for the most part extremely well hidden. For instance, Disc priests have to collect 12 books to make theirs which are in places as varied as a rare in Azsuna, Naxxramas and New Hearthglen in Northrend.
I have not heard as of yet any info about the Mage ones. The vanity texts on the items you use to unlock them seem to be hints of where they are. Though Mage ones aren't super helpful.
http://legion.wowhead.com/item=13955...mancers-charge Arcane
http://legion.wowhead.com/item=139559/the-stars-design Fire (this is the only one with a good hint IMO)
http://legion.wowhead.com/item=13956...urning-crystal Frost.
If you care to search for these on beta, you want to carry along an outlaw rogue as well because their find treasure ability works on other class' hidden tomes if they are lootable out in the world.