What kind of stock?
http://www.wgcshop.com/pcart/shopper...390_srch_ca870
http://www.wgcshop.com/pcart/shopper...-01_srch_ca870
http://www.wgcshop.com/pcart/shopper...386_srch_ca870
http://www.wgcshop.com/pcart/shopper...K07_srch_ca870
http://www.redwolfairsoft.com/redwol...l?prodID=22363
http://www.redwolfairsoft.com/redwol...l?prodID=17549
http://www.redwolfairsoft.com/redwol...l?prodID=17378
If you like the rail+M4 stock one, there's a cheaper one on WGC as well that I couldn't find at the moment. It doesn't take 12g CO2 though. I think the STAR one without the rail is cheapest of the three. Not sure...
For the CA870 and Tanaka M870, all of those should be as simple as screwing on with the stock screw. The Maruzen M870 and M1100 are a LOT more difficult, since they can only use stocks with a gas system built in (unless you go external), while the CA870 can use any stock, gas system or not.
*edit, was it me who asked this question some time ago?
I was looking for stock solutions as well for the M1100 rev3 a while back. I found a ton of CA870 stocks, but very very few M870/M1100 ones.
**edit, here's that other M4 style stock:
http://www.wgcshop.com/pcart/shopper...C-391_srch_870
apparently this is the cheapest one. Just tug out the gas canister/hose and sell it to someone.