A lot of first time Cannondale owners and others a like, may think there are no other options for parts for your Headshok's; however, there are more than one might think. Here I've listed some of the products I have come across and where you can find them.

Stems
Three companies make stems for the Headshok system: Profile, Syncros, and Kore. Kore offers only one of their stems for the Headshok, their top-of-the-line Kore Elite stem. Profile makes a stem in their Stiffy line for the Headshok. The stem offered by Syncros is the hinged face stem. The Kore stem can be purchased at Cambria Bicycles. Universal Cycles has the Profile in stem. I have yet to find a supplier of the Syncros stem that lists it on their webpage for purchase. Maybe someone knows where it can be had.

Adapters and Reducers
Headshoks are a great fork, but sometimes don't quit fit the job you need them to do, especially when it comes to longer travel. The solution is to use and aftermarket fork. To do this you either have to get a Headshok reducer and headset, or an headshok adapter. CLC Concepts makes an adapter that allows you to use an aftermarket shock and the headset already in the Cannondale frame without buying a headset. The reducers can be purchased from Universal Cycles.

Top Caps
Something new I just found are replacement caps and dials for the top of your Headshok. This is something to replace that cheap plastic cap, dial, or on-off lever. Only one company has thought about this, the company's name is CLC Concepts, you maybe have noticed their link on the main page.
Here is a picture example