Roof Rack universal clamps/clips - where to buy?

[Hyperloop]

Hello, I own a 2001 Yukon. The roof rack has rails down each side that the crossbars clamp in to. I would have to imagine that there are aftermarket clamps that are meant to utilize the same clamping system that the crossbars do but so far I am coming up empty. Anybody have any ideas? I may not be using the right terms when I am searching...