After you turn in 80 mini medals you can buy a from a list and the ore is 15. They drop from, my experiences so far, King Metal Slimes and Gem Slimes. Also they are possible contents of rank 9 and 10 chests in grottoes.
Yeah, I used to have a problem with Orichalcum. Now I have 56 mini-medals in my inventory. Just play through grottos and you will get them no problem. --- "You could kill babies in Dante's Inferno..." - poompop "Actually, you can kill them in real life, it's just kind of frowned upon." - rep_HC
It's a drop from Metal Slime Kings too, but probably the rare one. You'll possibly get a few just farming them if you find a good grotto level for them. --- Shameless plug for personal gaming blog : http://fk-nantonaku.blogspot.com