Where do you get supreme gems in final fantasy x?

Answer 1

You can get 60 Supreme Gems from the man at the Monster Arena after capturing 1 of each monster from 2 different species (Bombs, Flans, etc.).

There are also a few monsters and bosses that you can get Supreme Gems from:

Jormungand is a boss in the Monster Arena, unlocked after capturing 1 of each monster in the Djose Highroad. He will normally drop 2 Supreme Gems (he will rarely drop one Dark Matter instead).

The rare steal for Sin's Left Fin, Seymour Omnis (Within Sin), and Sin (last encounter during the airship portion) is 1 Supreme Gem.

You can bribe Halma (Omega Ruins) for 260,000 Gil to get 20 Supreme Gems.