You can pick up a full interior for 300-1000 depending... You just get involved with other Supra owners near you and watch the classifieds. Also, I have bought and sold about 6 Mk3's to help fund the project. If you know what your looking for you can buy, fix and sell. I made a few thousand profit doing this.
As well... You can find a rolling shell with a complete interior for cheap that could have a great interior.
Other than that, there are carbon fiber pieces available and you can buy the seat covers on eBay (custom leather) for about 800.00. You can also buy a pre pressed carpet replacement (not an exact fit but pretty damn close.
Also check the classified here as well. I have almost 2 full interiors now... Lol
Not done mine yet. Going T4 killed my interior/wheels budget.