Go to the Island Closest To Hell and defeat a Ruby Dragon (pretty hard unless you have good stats). It'll give you an Energy Crystal which can then be refined into Pulse Ammo. You can also get Energy Crystals from Elnoyles, which can be found at Esthar. Just run up and down the road ways a few times (on map scene) untill you meet one (They look like the guy you drew Siren from right at the beginning of disk one on the SeeD mission). They may take a while to come up but keep trying