I got a beartooth recoil pad / lop kit for my 870 express youth 20 gauge at gander mountain. The gun has a 13" lop and I have a 14"-ish lop. The kit I bought is a neoprene slip on pad with spacers of various sizes. My "new" shotgun now has a 14" lop. I can't wait for turkey season!
You can get the same kit at your local sporting goods store (Cabela's, GM, bass pro, etc) for less than $20.