Check out eBay, I have bought 870 wood at pretty good prices. You might even want to buy the complete set. Forearm and butt stock with a recoil pad. Check the butt stock screw length, I don't know if the 870 uses the same screw for both butt stocks.
Addendum:
Just now checked eBay, and found the prices higher then they were three months ago when I bought my last stock. Let the buyer beware, the closer it gets to hunting season, the higher the prices.