Have to say that this is something like I would expect to see on the cover of an epic fantasy novel As an aside, have you thought about advertising your services to that sector of the market? Probably a little more money in it than regular commissions?
Off the top of my head I don't know of any specific site, however perhaps talk to one of the publishing houses that handles/specialises in fantasy and science-fiction titles (Tor, Gollancz - however you spell that!). With your portfolio of commercial work and experience I'd have thought they would at least point you in the right direction of who to talk with. I'm aware that for some of the big name writers, it is the publishing companies that typically choose the artwork that graces the covers - the writer themselves doesn't always get a say in the matter from what I understand.