First place I could find is in the area of Fox Lair Pass for SCPD and the second race "Interceptor" titled Under Pressure. You will get to pick from rapid deployment group of cars. The only corvette is the z06. You can go under your garage and see how much bounty is needed at your current bounty to unlock if you do not have it unlocked. I would try to at least get a medal on busting the single racer. I ended with gold personally and I got the achievement once at the bounty screen. Hope this helps.
Also another location is in eagle Crest area and race name fight night. My buddy got it without getting gold on the mission.
Also another location. "Big Timber Forest to the list. It's the very bottom Time Trial named Slide Show. Gold is also not required as well."
Thx j furlong
You can also unlock in the "Fight Night" Interceptor event located in Eagle Crest once you have unlocked the Corvette ZR1 cop car.