Wait until chapter 5 go to Witherwood outside the save guy and grind there. there are fights that give 999 JP there. --- "The easiest way to stop piracy is ... by giving those people a service that's better than what they're receiving from the pirates." ~ Gabe Newell.
If you've just got the ship, then go to the only place in Eternia where you can land. During the day, you can find groups of like 6 or 7 mandragoras and they're worth quite a bit of JP --- Official Roof Fiddler and Maestro of the NDF 3DS FC: 0516-7288-9163 Handle: Brandon