I wouldn't say GSP cars are rare; there are plenty around. I think the most rare cars are the 4-eyed versions. That said, maybe some from smaller orders or from agencies that only used them for a year or two. There are a bunch of those listed in the usage chart compiled by C. Ricks that's posted over on .ORG
http://www.sspmustang.org/technical/vehicle_usage.htm
Keep in mind that not everything in there is correct (or up to date), but it should give you a good idea.