Yes they are specific to the car. IRS Celica's would have similar springs, however I expect the spring rates would be different. Call a few suspension shops in your area to see if they can get new ones for you, otherwise you can just go to a springworks and get new springs made. You'd have to find the factory spring specs first, but I'm sure they're available somewhere.
Note that while there is no factory rear camber adjustment on a Mk2 Supra, it is possible to adjust the camber by adjusting the height of the rear subframe relative to the body. There are 4 attachment points on the subframe, and they are insulated by large rubber bushes. If you cut these bushes down or remove them entirely, this will result in more positive camber. Conversely, you can add extra spacers in there to get more negative camber.