no oh god no, not unless you wanna die in the process!

Ok firstly try fighting the Elnoyle in Esthar with Rare Item ability from Bahamut equipped and then you should get 2 for winning and another for mugging it.

Other than that if you are level 80+ then fight the ruby dragons on the Esthar hill tops or Island closest to Hell (Island closest to Hell will contain level 100 rubies, damn hard)

Or the non combative way (no level up game or just too peaceful for the storyline) is to play triple triad with players that carry the elnoyle card (any of the higher cc members) and when you win ALWAYS take the elnoyle card if they play it. 10 of these equate to 1 crystal.

Of the above, I recomend Elnoyle. (trust the guy with over 500 hours playing the game to make sure he had everything! maxed out)