Best success I've had finding some stuff has been eBay surprisingly.
There is a L/H rear quarter and a rear panel (both genuine NOS GM) on there at the moment. Whack in these listing numbers into the ebay search bar 324098010685 (rear quarter) and 254194309695 (back panel). Both aren't "cheap", but perhaps be a little cheeky and see if they'll take a bid on them?
The shop selling the back panel does have the make an offer button. I have bought a NOS drivers sill from them and got money off. But they use Hermes to deliver stuff, if I had known beforehand I'd of paid a little more for delivery. Luckily it turned up and wasn't damaged.
Anything else panel wise. Search German eBay and some online "youngtimer" Opel places (COPE, MM Opel parts and Classic Opel parts). My harder to find panels I've got for my Saloon have came from Europe. Some stuff wasn't badly priced. Delivery can be a killer though.