You can find it here:
http://shop.shapeoko.com/collections/fr ... z-axis-kit
It moves the Z axis motor back giving you more clearance for your spindle. It also features a 2:1 ratio so that you can turn up the speed on your Z axis.

EDIT: There will be a NEMA 17 version as well I am just waiting on the belts to get here! I expect them within a week.