I guess it must be just luck of the draw.

Purifying Salt*
Usage- damages & magic effect cancelled a single enemy
- 30 for No Encounter in armor
- 3 to teach an aeon Dispel
Steal- Abadon (Monster Arena)
S. Rare- Warrior Monk (Holy Bevelle Temple)
- Fallen Monk (Yevon Dome)
- Hornet (Monster Arena)
Drop- N/A
D. Rare- N/A
Bribe- N/A
* 99 for catching 4 of every dragons

I'd try getting 4 dragons, shouldn't be too difficult if you know where to find them.