GAHH in LA is a big supplier, as well as World and others on vendor list; there are a few Euro suppliers too
IMHO...its not a DIY job; even experienced SL mechanics have local convertible top places install them; special tools, tricks and missing straps/seals can sabotage a job ans then you have to cut it off frame.
I would check locally who does convertible tops; they get wholesale rate from Gahh and vendors....
ps, unless ur old top is in shreds; a good washing with cleaning supplies from Ragg Topp and polishing plastic window, and gluing in new rubber seals or straps can save a lot.