Sounds like your interested in Limbus.
Basically:
1) Buy detergent
2) Tele out to crystal and enter Sea.
3) Kill the flower for Card KI.
4) Enter zone your aiming for
5) ???
6) Profit
Most everything in Limbus drops at least 1 coin, frequently. Chests can drop a full pool's worth sometimes (Usually 2-5 though).
Also, you may be able to find people just bazaaring ABC's, but prices tend to vary ALOT. On Bismarck I've seen as low as 1,000gil per and as high as 20,000gil per (LOL).
I bought almost all of mine, just a few here and there, until I got my Brutal/Loquacious Earrings. All I wanted. :D