I think you need to be a high level, or a high alteration skill level for it to be stocked.
There is one guaranteed copy that isn't too hard to get though. Between Riften and Ivarstead, there's a camp next to a fort. Talk to the guy at the camp and he'll send you on an infiltration mission. The reward is a Detect Life spell tome.
Have you traveled the Rift extensively? Between Rrrrriften and Iverstead is a fort that has 3 bandit looking people sitting around a campfire. The guy's name is Stalleo(?) He asks for help clearing out the fort. Follow the instructions, complete the quest, and you should receive a detect life spell book.
It's not Falldir's Tooth or whatever, it's further west than that.
Good luck, I hope that helped... --- Redguards Rule!