tein s-tech springs for the ts is actually the s-tech for STI. My 2004 ts uses s-tech springs. These are too soft for STI actually but just nice for the TS.
There are no specially made springs for the TS, but they work just nice as the TS is almost 150kg lighter than the STI so the spring rates are just nice for the TS.
i dont know where you can order them from, but i think any tein dealer can help you get the STI S-TECH.
Everything from the WRX GD series should be able to fit. Subaru makes it all interchangeable. You can swop everything -shocks, springs, links, lower arms etc. just make sure if your version is the blobeye ( GD applied C onwards) for the parts to fit. Bugeyes may have slight differences in the mountings for the shocks.
regards