Dear Rollaround, Keith, Mark, Buick_65, and mid-60s Buick
"cover-ups" . . . .

Get to be that time of year again. Anyone have a good source for a quality car cover that fits our 65s?
You certainly would be making a safe choice with California Car Cover, but I must confess that before I knew better I bought car covers from J.C. Whitney and they fit my 1965 Buick Special wagon very nicely. I don't know why, but I think there are very detailed patterns out there and fit isn't likely to be an issue.
There are many different styles of car covers. Of course by far the best style is a -
garage! 
When my wagon had the misfortune of having to spend time outside, I went with the evolution-4 type of fabric that was very heavy-duty and at least appeared to provide some protection from the elements.
Fortunately, I was living in San Diego at the time, so rain events were rare. Even so, I went to the extreme of keeping an old car cover and sandwiching between the new car cover and the old - a plastic tarp. Once the rain event had passed, I would remove everything and make sure the car had a chance to dry completely before putting the new car cover back. Yes, it was an extreme regime, but my wagon has very little rust even if she spent a few years outside in San Diego.
So take a little time to see what your fabric options are and choose something that makes sense for your conditions.
Cheers, Edouard