Real steel synthetic RPK stocks are pretty rare. They only made plum stocks that fit the standard tang, all of the black ones were made for the sidefolding variant iirc. Nobody will have a consistent stock of them. I've only seen a few plum stocks pop up on various forum classifieds in the past year or two and they generally cost upwards of $200. Given the way the AK surplus market is going, I wouldn't be surprised to see them break $300 the next time a few become available. I've never seen any RPK folding stock hardware or furniture for sale.
iirc Molot is getting into airsoft and might release a poly RPK-74 in the future. Might be worth waiting for as the gun will probably be just as expensive as the stock you want.