Advanced Technologies Inc. (ATI). They make a side folding stock which works, it lines your eye up over the barrel and provides adequate drop and length. If you want to be able to fire the gun with the stock folded, you must use the included fore end, and modify it with a file. The fore end doesn't fit well at all, and will probably require you take a file to it anyway. The stock itself fit very well, the only work required was to trim the moulding flashing down and eliminate a few sharp edges. The butt area is very small, about 4 1/2 sq. inches and hard, so the optional extra cost butt pad is recommended.
Other side folding stocks include the Butler Creek plastic folding stock (useless), the Butler Creek unit made from metal tubing (a little better) and there is a side folder from Choate which looks like the ATI (I never got to see one).
P&D Enterprises in Edmonton had no less than 4 folders for the 870 in stock when I was looking, including a top folder.
Sharptail