I've seen the original-style clamps still listed on the roverconnection.com website (so they may be available even if the racks themselves are not), and I can also attest to the ones that Derek Chase (Urban Offroad) is making for his Safety Devices-style roof racks. I recenty bought one of his complete racks, and am very impressed with his workmanship and the rack's quality. His clamps are a slightly simpler design that the originals, but work great. I'd recommend looking into either of those options.
Good luck with the search.
'86 D110 3-door
'97 D90SW - SOLD SEP '14
'94 D90ST - SOLD MAY '13
D90s sold while assigned overseas. You're welcome America!
'06 Range Rover Sport Supercharged
'8X FankenRover D100