Fight Elvorets. Go to the shopping mall in Esthar, go north, go north again, then go left. talk to the guy in black, and you'll fight an Elvoret. They give you 2-3 crystals each fight. You can fight him as much as you want, just go back an area, then go back to the area you fought him. Use Ifrits ammo-making ability, and you get 10 pulse ammos per crystal.