I'm sorry to report that FMA is no longer carrying Z sensors. They ran out of existing stock and because their basic Co-Pilot doesn't require them, they won't be making any more. They will continue to make XY sensors, which the Co-Pilot does use, and those should be back in stock in a few days.
We're currently exploring alternative sources for the Z sensors, but in the meantime, I suggest you buy them from Dean Goedde for $40 including cable (scroll to bottom for price list).[Update: Turns out that those may not be totally compatible (they're 3.3v and require special wiring to our 3.3v shield). Stay tuned....we're looking at other options.]
[Update 2: Dean writes and says that they are compatible: "My Z head is 100% fine at 5V. The analog output will be 1/2Vdd for a balanced horizon view, and skew away based on tilt. After all, myself, FMA, and Paparazzi are all using the same components and design. The AD8551 OpAmp is being used in all 3 of our Z heads and is fine to at least 5 volts.]