Advancing in OutfitterFollow

How do you advance past lvl 19 outfitter? Do you have to choose a subprofession, for example: Armorer, Weaponsmith, or Tailor? I know you can buy the books from a vendor, but where are they located? Ive looked up the books but all it tells you is who has them, and when i search the people that sell it it doesnt tell you where they're located? Do i have to go the an advanced crafting trainer to choose a subprofession just like when i had to choose to be an outfitter? Im pretty sure im not the only one who'd like to know and i pretty sure there are other threads about this but im not going to go searching for them waste of time since im not a premium.