Are you thinking of Coast Car Covers down on Morena Blvd. maybe? I think they used to advertise in the WW.
I just bought a cheap cover that I think would fit your car off evilbay for $60 from CSC Sport. It's
a 4-layer, laminated fabric one for a '74-89 911 w/ whale tail. Supposedly 100% waterproof yet "breathable." They also have a 5-layer product for
$75 that is for the same series car. They shipped super-promptly and it got here in a few days because they're in CA, which unfortunately also means you have to pay sales tax. I think the "whale tail" bulge in the rear will help it fit w/ your upper wing element mounted, but it would probably hike it up in the rear a bit much and maybe not cover the bumper and rear wheels well? You'd probably need a custom job to really fit perfectly, but if you want to try this one out on your car, come on over and borrow it for a test fit.
BTW, the company I told you about at the AX that might have the SS buttonhead machine screws you needed is
Maryland Metric. A "senior moment" prevented me from remembering the name when we were talking about it, so I looked it up when I got home.
HTH,
TT