How do I get Safecrackin'?
Vendors will occasionally sell a skillbook. Some of these books teach class skills, others teach misc skills. I learned safecrackin from a book I bought from Wanderin' Sally. I've seen other books sold by the main store in Dirtwater.
The safecracking book is called Get Crackin': A Guide to Modern Safes. (thanks @yuuki)
Also note that buying multiples of the same skillbook does not increase your level in the skill, you have to use XP for that.