Hi j.brendan.hagan,
We do not sell the Jetson Xavier NX Developer Kit carrier board (P3509) individually.
You could only buy the one devkit + Jetson Xavier NX Module (P3668-0001) (Production version), or you can find other carrier board from our HW partners: https://developer.nvidia.com/embedded/community/jetson-partner-products?t1_supported-jetson=Xavier+NX