If once you started to investigate for cars' real performances data you might have stumpled upon automobile-catalogue.com from Pawel Zal, a very detailed source of car specification data.
Just a hint: If a car's engine has no given redline (in the 'powertrain' tab of 'full specification' -> 1st pic) simply divide the 'theoretical top gear speed' of the car's highest gear ('drivetrain tab -> pic2) by the 1000rpm gearspeed value of the same gear, multiply the value by 1000 and you get the approximate redline value.
For example the 1978 Lamborghini Countach LP400 S:
No redline given, top (5th) gearspeed (theoretically): 315 km/h, 1000rpm value (5th gear also): 37 km/h
-> 315/37 = 8.5¯135 x 1000 ~ 8.500 rpm, that's your redline.
Greetings
Frank