Welcome to the site,
You might try Numrich Firearms at Welcome to Numrich Gun Parts Corp.
, your paticular shotgun might not be listed but you can request parts information via a forum or parts information request.
The information I have on your shotgun is : Nitro Hunter
,Trade name used by the H. & D. Folsom Arms Company on shotguns made for the Belknap Hardware Company of Louisville, Kentucky.
There's a very good chance that the Shotgun was made by Cresent Firearms. I have seen this trade name (Nitro Hunter) associated with the Savage Company on the internet, mainly because Savage bought out Cresent around 1931. I have seen posts on other sites where a model of Savage was crossed over to the Nitro Hunter, I'm not sure that this correct, Savage built guns on the remaining Crescent parts and they were listed as being marked "Cresent" only. This is where Numrich comes in, they would know if parts cross over.
If your stock is beyond repair and you are unable to find a replacement, you might consider checking the internet for a costum stock maker. Another source might be the auction sites such as ebay, gunbroker.com.
Sorry I don't have a slam dunk answer, but the information concerning these shotguns tends to be very vague. Numrich has been thru this before, hopefully they can get you a part, or a source for one.
If I find any additional information I'll post, somewhere I do have a old book on these shotguns just can't find it. If anyone has any information please post.