I haven’t found information about recoil pads compatible with Knoxx stock and have sent this question to Blackhawk.

So, if you want to replace a standard recoil pad on your Knoxx Spec Ops stock, you can get LimbSaver recoil pad which they offer (part # is K18005-B).

Related post: Sims Vibration Laboratories LimbSaver Recoil Pad for Remington 870