They're both 1983 cars, and thus (as non-limited production models) can be bought from the Used Car Dealer when in stock.
Currently they are not in stock. However one is available as a reward car in Menu Book 10 and the other is available as a reward car from playing the free My First Gran Turismo demo - which takes about 90 minutes if you're being leisurely about it.
The cars come as standard with a naturally aspirated engine. If you have a turbocharged engine it's because you've either fitted a turbocharger to it or you've done an engine swap (as both cars can accept the turbocharged Toyota MR2 engine). You can undo the turbocharger by removing it in car settings (again, basic, basic stuff) but the engine swap cannot be removed.