At least for the DMM version, according to the wiki, the gem gacha originally had Urania, Rusalka, Boreas, Perun, Indra, and Nergal. Then about 3 months after launch, Orpheus, Mephistopheles, Astaroth, Undine, Zephyrus, and Baphomet were added. So that'd be the original set of R kamihime. No other R kamihime is either in the gem gacha, or has a rate so low that no wiki user has seen her.

So, yes, jewels are the primary way to pick up new R kamihime. Also, starting with Ixion, advent type events will have a kamihime weapon ticket available for exchange. I think that it includes R rarity, since it doesn't specify 'SR or higher'. Ixion itself will be the 6th event after Typhon.