Besides spending money, or converting ironite to FC...

I've used a bunch of daily dungeon keys, and that's probably the best way I've found to get FC, without a net loss of ironite.
Some frontier dungeons also resulted in occasional 15 FC drops, I'm unsure if there is a dungeon that drops more FC than others. (I've got around 60 FC from 2h running the infernal dungeon, with no loss of ironite). Perhaps other dungeons give more? There is a bunch of dungeons I've have not tried yet... cosmic talisman, frozen wasteland, weekly challenge. And others I cannot remember if they dropped FC; have a bunch of keys for all.

Would like to buy Horus before attempting to run Powerslave, and I'm 800 FC away.
Thanks troopers!