You can find out where any Skills are learned by clicking on the Character button on the upper right corner of the screen, select Skills, and then click on either the Adventure, Trade, or Battle tab. Select the Skill you wish to learn, then click on the Location tab on the lower right corner. You will see the location(s) where it's learned at and the NPC(s) that teaches it.

Note that if you are in a job that has that Skill as favored, you can learn that Skill in ANY Cities that contains a Guild Master with the Guild Master himself.