I seem to recall that there's a very limited number that can be found via one-time quests & the like, unless that's been changed.

If they're still hard to get then I suggest they are made craftable or as drops with these changes:

1) Identical to current use but unique tag removed.

2) Made into real Phoenix Down - can be used to revive another character PLUS has the ability to revive the carrier upon death in solo content only . Keep the unique tag to stop people just stacking them to cheese content.

If they become craftable then I'm tempted to say they should be unsellable. The recipe would need to be reasonably easy & not using rare materials but only possible after a certain quest line (DoW/DoM/DoH) at lvl50. That way being a crafter isn't essential so raiders won't complain & everyone who reaches lvl50 can craft their own.