MPEP 2232.01 (https://mpep.uspto.gov/RDMS/MPEP/E8r8#/E8r8/d0e224037.html)
IPX has been replaced by IPR (Inter Partes Review). So, IPX is no longer available for patent challengers. However, there are some instances I had to look up IPX histories of patents at issues in litigations.
So, go to the Public PAIR of the USPTO and enter the patent number. In the ‘continuity’ menu of the patent’s page, find a continuation with the number “95.” MPEP 2232.01 states, “Scroll to “Child Continuity Data” where any related reexamination will be listed. Ex parte reexaminations are identified by the unique “90” series code, e.g., 90/005,727. Inter partes reexaminations are identified by the unique “95” series code, e.g., 95/000,001.”