Leupold makes a 0.54" riser (part of the integral mounting systems), in versions using either screw attachment or the 0.5" nut attachment (Mk1 and Mk2, respectively). I've seen that Al Flahertys carries them.
Brownells makes a 0.75" riser (one piece), as well as a base that can be paired with different riser heights (for 0.75" or higher). Called the AR-15 flattop riser, and AR-15 modular flattop riser mount system, respectively. I've got one of the former on order. It attaches to the rail using the 0.5" nuts, but they should only be tightened to 20 in lbs (so not as ultra skookum as other nut attachment type mounts that take 55 in lbs, like ATRS rings for example, but should still be good).