Must go to the Roush webiste to buy the springs.
http://www.roushperf.com/catalog/index.htm
Check the Cougar catalog.
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Koni's will improve your setup immensely over stock struts, if you are happy with the reviews of the Roush spring, then get them. If not check out Eibach, HR or BAT springs (Intrax may still be making springs for car as well). I think the spring decision is a tough one as they all asthetically look different, but generally behave the same.

Money doesn't always bring happiness. People with ten million dollars are no happier than people with nine million dollars ~ Hobart Brown