PXI is a standard that is based off of the CompactPCI specification.
Page 41 of the PXI Specification Manual
outlines the J1 and J2 connectors on the back of PXI cards and what signals are assigned to these groupings. National Instruments PXI peripheral cards adhere to this standard. The connectors can be found by searching for "CompactPCI connectors" and can be purchased off many 3rd party sellers.
Additionally, the dimensions of PXI cards that fit within a single slot in a National Instruments chassis are given in the document below:Dimensions of PXI Cards That Fit Into Single PXI Slots