Check out these two hyperlinks that a member of this forum using the screen name T. O'Heir put me onto when I was locking for a replacement forearm for my Model 11:
http://www.gun-parts.com/remingtonshotgun/
and
http://www.gun-parts.com/remingtonstocks/
Note: If you're interested in the stocks, pay close attention because there are apparently several different tang sizes and different screw hole arrangements. You'll need to measure both tangs and know where the screwholes are supposed to go in order to make sure you get the right stock for your particular gun.
As far as internal mechanical parts of the gun are concerned, two of the most likely parts to ever need replacing are the firing pin and the action spring. Midway USA listed both of these Model 11 parts for sale in their Midway Master Catalog #32 which I received six months or so ago. They listed the Remington Model 11 Extra Power Action Spring (from Wolff Gunsprings) as their product # 763-504 for $9.99. They listed the Remington Model 11 firing pin (by Glend Arms) as Product # 233-288 for $14.49 ( but be careful when ordering because I think some Model 11's had round firing pins and others square and I don't know which one the advertised one is, or which style your gun uses).
Midway's website is
www.midwayusa.com