Kill sacrifices for perfumes and sell them. Sacrifices are the white cie'th in the last like, 2 chapters.
Also, riding Chocobos (get the reins from Mission 14) can sometimes let you find treasure that can be sold.
If you look in your inventory, anything that say's "can be sold" will net you gil. Selling those items is what they were created for...not upgrading your armor/weapons.