Right now there is no other option to get Orichalcum. When we get the next Disastre Difficulty, there is chance that those drop Ori fragments which can be traded in for Orichalcum.

Personally, if you main Thunder I would use 3 of them and FLB them since you then gain a quite good chance for crits aswell as having flat 15% Assault, which not many weapons can reach that easy. Additionally the extra stats you gain aswell. Costly, yea, but you have to start somewhere to get stronger.