I'm looking for a side saddle and possibly a pistol grip stock. All I can readily find is stuff for 870's, Mossy's and Win pumps.
Best source for shotgun accessories is Brownell's:
Since the Winchester auto was never popular as a defense gun, and the FN hasn't sold a significant number yet, there are few to no accessories available for it.
Speedfeed MAY make a stock to fit:
Choate makes more accessories than anyone. You may have to email them for a definitive answer.