All the ones I see are factory lately. Browning for instance makes low and high rib models in some guns and high ribs for skeet specific and trap specific guns. Simons and Poly choke used to make them for converting plain barrels back some years ago. Not sure if they still have them. You could contact Poly choke as they have a new owner and I was just on their site a while ago. Didn't notice if they still had ribs. You may want to contact your gun's mfr and see if their custom shop will do it. I think you could get a high rib conversion from Browning. There was a rib system that I saw on a couple of guns some time ago that looked like it stacked on a stock rig, but I can't remember who made it.
More ammo! I need to shoot something!