Might be hard to find. If I remember correctly I had these same racks on my Supra. They used to fall off all the time because the clamping system just didnt work. Mine got sent into Mako by the dealer and the clamping system was replaced with one that threads on.
Where exactly did you get those without the clamps. There is a reason it is hard to find them. Because monster doesn't like selling the clamps so people can just run up and grab the rack off somebody's boat and just order the clamp.
I just bought a 01 SAN last weekend, and the guy i bought it from had them from a previous boat. He said he couldnt find clamps so i figured i could find/make some so i bought them from him.