Virtually any electronic hobby store will sell you this stuff, it's used to make printed circuit boards. Or, use fiberglass or whatever. BUT, as Carl said above, unless you are able to exactly duplicate (material, thickness, area, of) the capacitor, you will need some test equipment to tune the antenna to resonance and the capacitor to proper phase match.
I like Carl's idea of mounting it to an aluminum angle attached to the rib, although I captured mine under the wingtip nut plate rivets, and left enough slack in the coax to allow removal and laying the tip on top of the wing.
I suspect most reports of poor performance come from people who did not do proper tuning. (Not counting the grossly incorrect installations!)