Obviously, sold by the JC Vendor in exchange for doing the daily quests.
This is probably one of the best gem's currently for a Prot Paladin, as Mastery is our primary stat. If you are not concerned about stamina so much (which is still a debate item as of this writing -- so long as you can survive 3 unmitigated hits) then this is likely the best gem for reducing your DTPS (Damage taken per second) while still allowing you to get set bonuses (mainly aiming for the +Stamina ones, to offset the stamina loss from using this gem)
Remember that stamina-stacking is a bit less effective with higher health pools, a 346+ tank will run 150k+ HP in raids, which is about that borderline to survive 3 unmitigated, unabsorbed, no-CD hits.
Best red gem for prot warrs?