Master Cards and Mastery Items are given by Legendary Smith after reaching job level 99. Each job has its unique item and can only be obtained one from the smith. Only the job itself can use its own item.
Once you reach job level 99, you need to find the Legendary Smith; her location is random though. Once you find hertalk to her while using the job.
Below is her locations where she appears randomly:
- Ur: Inside the well.
- Castle Sasune: The East Tower's third floor
- Healing Copse
- Village of the Ancients: inside The Inn.
- Gysahl: Inside the Chocobo stable.
- Dwarven Hollows: The entrance to the Subterranean Lake.
- Replito: At northern house, there is a hidden door behind the house.
- Saronia Castle: The hidden passageway in the left area where the two Black Mages are.
- Doga's Village: in the middle island.
- Ancient Ruins: inside The Inn.
Finding in these random locations is a pain in the ass. Here is a much easier strategy: since her appearance is random, you can always in/out between 2 of the above places. Luckily, Healing Copse and Gysahl are the closest to each other, all you need to do is fly between these locations.