Brownells has a pretty wide variety of shotgun stuff. Try CDNNsports.com, I buy parts from them on occasion and they are great company to buy from. Do a search on google for shotgun stocks, you'll come up with too many results to list.
IMO, you would want to stay away from the factory folder, I got one bought it new, and it has had quite a bit of play in from the start. If I didnt have a heat sheild on it I would go with the topfolder JMO