In the first level, after meeting Reeve and leaving the house (the start of mission 'Assist the WRO') the 5 to 6 DeepGround soldiers you fight there will continually respawn until you head toward the gates with the Mako points.

Between selling the potions they drop, and the experience you can convert to gold (or let them kill you, add to total, then lather-rinse-repeat) you can build as much Gil as you want. Makes buying the final weapon upgrades much easier, as well as maxxing out one or all three of every type of gun.