Being the power requirements and Tip/Ring are identical to the Pro-96 and the fact the Pro-96 AC adapter works on the PSR-500, I would assume the Pro-96 car adapter would work as well.
I really like the one that Advanced Specialties sells. It's a Monster brand with adaptors and switchable power output. They sell it for about 18 dollars. I've seen the same thing on several other sites for 30 bucks. Click on the link below, then "scanners", and scroll down till you see it. I use one to power my PSR-500.
Found a regulated one at Target yesterday for 10 bucks. Works like a charm. Not quite as a small plug as the Monster brand, but a small setup nevertheless. I forget the brand, but it was some Target house brand.