If you are just looking for a spring, try HyperPro. This company is out of the the Netherlands. They appear to have a lot of participation in the world racing circuits. Their approach is purely progressive spring oriented. They produce fork springs, rear shock springs, steering dampers, and possibly rear shocks. The US distributor is at HyperProUSA.com (I believe). HyperPro has specific fork and rear springs for the 05+ ST. The install kit comes with the springs and new fork oil--just drop them in. There is also a combination kit available to replace the fork springs and the rear spring. I have ordered this kit and will install it soon. I otherwise do not have any direct experience with performance. But, since I really dislike the front suspension under many different driving conditions, the forks being undersprung being at the top of the list, I thought I'd give this a try (relatively inexpensive and easy to do--do not have to detach the forks and send them anywhere.)
The HyperProUSA site does not list the specific part numbers, but they do have them available. They are available for telephone consultation.
One note of caution with the rear spring. Make sure that you or the installer know how to and have the proper tools to compress the springs for removal and installation. I don't claim to know the process precisely, only that you need a specific clamp or an otherwise effective process (strapping, etc.).
The other option may be RaceTech, though I do not know for sure with regard to rear spring only replacement. You will have to give them a call and chat with a technician. RaceTech does not use a progressive spring approach.
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