If you can take the time to have your car "down" for a week or so, the friction disc and pressure plate can be rebuilt by most any clutch and brake rebuilder. Ask your local auto parts supplier for a reference. Then get your flywheel re-faced at a local machine shop. Just make sure they remove the same thickness from the driven face and the "shoulder", since it is a stepped flywheel. Decent shops will know to do this.
Or, you can order whatever you need from Opel GT Source (http://www.opelgtsource.com/
). They sell both aftermarket and OEM friction discs, as well as the pressure plate and release bearing.
Or, you can just go to your local auto parts supplier, and see if they have a listing. There is a terrific cross reference spreadsheet available for download on the classicopels site at http://groups.yahoo.com/group/classicopels/files/GT Parts Oct 2002.xls
which lists a couple of AutoSpeciality and Beck/Arnley parts numbers for all the required parts.
Finally, if you want to upgrade your clutch (if you have hotted up your engine) you might want to consider converting to a Chev S-10 clutch. It requires a machine shop to flat-face the stock Opel flywheel and re-drill it for the S-10 pressure plate, and some minor clearance grinding on the bellhousing. But that's about all from what I have been told (I am still a "purist", and went with option #1 above). I think it even uses the stock release bearing.
Speaking of which, have a critical look at your release bearing, since this is the time to replace it. And also your pilot bearing, which is the needle bearing in the crank end that the transmission shaft rotates in when your clutch is released. At a minimum, make sure this bearing is cleaned and re-greased.