On the cheap
As mentioned Brownell's has a couple different kinds...
But being the cheapskate that I am (and perpetually broke lately) I made my own...velcro on a recoil pad I had laying around, as well as on butt of stock...this gives me horizontal and vertical movement (and actually works well, stays right where I put it)...With some creativity you could add/ subtract layers to alter LOP as well
Just a thought.