http://www.blackhawk.com/catalog/Shotgun-Stocks,165.htm
The second page of items includes Spec Ops stocks, which might fit what you're looking for.
A couple things to keep in mind with those stocks:
- It's a little harder to use the safety button and maybe a little harder to push the slide release button with a pistol grip stock than with a conventional stock.
- I've heard that the SpecOps stocks tend to put the user's eye higher than normal, so it may be harder to sight directly down the barrel.