Numrich is always a good source.
www.e-gunparts.com
I've also had good luck with Dick Williams. I had an old Montgomery Wards shotgun that wouldn't go. I could tell it was missing a piece in the trigger assembly but didn't know what. Sent them a picture and talked to them on the phone, 15 bucks later I had the parts.
http://dickwilliamsgunshop.com.