The OEM folding adapter is made by iTac, which is a SIG-owned subsidiary that manufactures/imports firearm accessories and parts from China, as best as I can tell.
The AR-threaded folding adaptor can be ordered directly off of the Sig website, although to be perfectly honest I don't know why you'd want it, as it's unequivocably a piece of crap. The newer production pieces have had fewer problems, but they're still widely known for breaking in various ways.
Go buy an Ace adaptor like the one shown above, and stick a CTR or MOE stock on it.