Take detailed measurements of the spring. Be as accurate as possible. Then go to the following website and find the closest match:
www.centuryspring.com
If you don't feel like ordering $40 worth of that spring, let me know and I'll throw a couple in with my next spring order. It'll save you the trouble of setting up an account too. Since I'm feeling generous, if you can't measure the spring yourself, you can send it to me and I'll spec one out for you. Post a picture first though so I can be sure it's not some wild custom design.