You can get springs at Summit, Jegs, Speedway Motors, Aldan Eagle ... Assume you are looking for coilover springs - Hypercoil, QA1, etc. eBay also works.
http://www.speedwaymotors.com
http://www.summitracing.com
http://www.jegs.com
You should know the inside diameter, 2", 2-1/4" or 2-1/2" and length, 8", 9", 10" ...
Try the zip tie on the shock shaft and see how far it moves during the bumps and see if it really bottoms out ...
Springs are cheap, but your ride might become harsh.
There are other threads on this site with shock/spring calculations ... if you know corner weights, angles, etc. try this site. Good background information.
http://www.swayaway.com/TechRoom.php