Assuming you're from the west side of the Atlantic since you mention GTA, I'm afraid I can't help you with US sources. Maybe Joe Wagner can?
Things aren't too rosy in Europe either, since the R9/Alliance was never a popular tuning object and it's 15 years since these cars were new. But there's still something available. I'm not aware GTA front coil characteristics, but I'd assume European Renault 9 Turbo (or 11 Turbo = Encore) front coils might be pretty close, as I've understood the GTA is rather stiffly sprung.
To get a feeling how much the Turbo lowering coils will drop the car, have a look at my champagne coloured Renault 9 here:
http://netti.nic.fi/~gordini/Renault9/freshalloys.html. I've fitted such coils to it. I don't remember what manufacturer they are from, but they were said to give a 40(ish) mm drop compared to a standard-height R9 Turbo. It says -50 mm drop on that page, but it's compared to my car's original height (standard Turbos were already 10-15 mm lower than non-Turbo cars like mine).
Some European websites where you should be able to find R9/11 lowering springs:
http://www.ompracing.it (at least in 2001 they still listed R11 Turbo lowering coils, maybe they even have an importer in the USA as OMP is such a big name in motorsports)
https://www.needforspeed.co.uk (a very comprehensive site with lots of articles. I've seen R9/11 lowering coils here, can't remember if they were for the Turbo. Mail delivery worldwide I guess)
http://www.cesam.com (they're a French tuning company so they probably don't spoeak English?)
Hope this helps some...