General

Or just mine it

You should go to places like MP3 and Showa town. You can find heartstones and (legendary) mysterious veins which can even give cubes.

You should do part-time jobs on the character select screen. Every 6 hours, each character doing it (max of 3 per account) gets 30 ores each. So basically you can get up to 360 ores per day. The ores are random though. You can also get a chance to get a recipe for an Angelic Blessing Ring.