Touring Supra

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The MKIII never came with air dams. If you're asking about the front undercovers your only options are the dealership or used. Air dams you'd have to fabricate, there's not a lot to them and you can look at many modern vehicles to see how they're employed and to get an idea of placement. They're just rigid plastic sheets which extend down below the car to divert air from going under the car and away from the tires. Even Prius have them.

EDIT: Air dams for tires look like small (usually hidden) reversed mud flaps for the front wheels.

For turning performance a much stronger anti-sway bar will make a very big difference without sacrificing ride quality in a straight line. It does nothing for stabilization at high speeds though... different problem, different solution. Naturally, stiffer suspension will help too but the ride will get a bit rougher.