Where can I find a 10 N/m spring for harmonic motion experiments?

The simple harmonic motion experiment (activity 15) in Physics with Vernier, order code PWV, $48 is best done with a spring such as Sargent Welch Cenco's Harmonic Motion Spring.

See http://sargentwelch.com/harmonic-motion-spring-cenco/p/IG0038464/

part number CP75490-00

This spring is quite flexible, and is highly Hookean, unlike most springs. The resulting motion is much closer to true simple harmonic motion than is obtained using a cheaper spring.