Sadly, there are no easier ways to obtain money other than selling loot. Though I do recommend doing the Mark Hunts as they reward you with better gear by mid-game so it starts to go a bit easier on your wallet. You may also want to stop using Teleport Crystals and just start walking everywhere so you can level and get better loot.
Now may also be the time to start doing some of the side-quests to obtain the optional Espers. A good chunk of them should be available and they are some pretty awesome battles that will make you really have to use the Gambit system proper.