The only place I can think of that would be your for sure first call (other than looking in the classifieds for used parts of course) would be Rampage themselves. They are the keeper-of-the-flame so to speak for the old Kayline/STC tops.
The parts are not officially sold separately, but every once in awhile I hear about someone calling them just at the right time when they have some spare parts laying around.
It's a crap shoot, but well worth a phone call.
I have also heard of the occasional vendor having some spares. I think the last time it was Tom's Bronco Parts, but here again, worth a phone call to just about anyone.
Including us in fact. I'm only 97% certain that we do not have that part, nor have we ever had more than just a smattering of extra parts here and there over the years. But you never know, so here again, worth a call.
Good luck is all I can say for sure!
Paul