The simple harmonic motion experiment (activity 15) in Physics with Vernier (PWV) is best done with a spring such as the Eisco Harmonic Motion Spring. Fisher is one vendor for this spring.

This spring, a truncated cone shape rather than a straight-sided helix, is highly Hookian, unlike most springs. The local spring constant varies slightly along the length of the spring due to the slightly changing diameter. Our experiments show that the resulting motion is much closer to true simple harmonic motion than is obtained using a helical spring, especially when plotting the acceleration of the mass. Typically the larger diameter is used on the lower end of the setup.

There have been articles about these and similar springs in The Physics Teacher. See, for example,

The Physics Teacher 38, 228 (2000): A conical spring—Which end up?