The below site will help you find what you need. There is a vendor there for a new "tramp" (canvas platform), that may save you money over a used one (and last longer).
http://www.cat-alist.com/tobuysell.htm
I had my tramp re-stitched by an awning repair shop, but the stitching (his top grade) wasn't up to the elements and stresses. Wasted money.
Wolfeboroonline.com advertised an entire, complete Hobie for $500 last fall.
(It's worth triple that in parts -- like the trailer it came with).
If that cat is still available (but not currently advertised), try a "wanted" ad in the classifieds (here) or advertise (Hobie wanted -- for parts) in a Lakes Region paper.