Hi Charles!
Those are not for sale (well at least, I can't find it as well).
I think your only option is to buy a broken motherboard (RROD motherboard for example). And desolder the USB ports from there and solder it to your working motherboard.
Example: https://www.ebay.com/itm/226205910019?_nkw=Broken+xbox+360+motherboard&itmmeta=01J5AD04FRSN2MKEP2X9NHG621&hash=item34aaec4403:g:vPIAAOSwYB9mdbMS&itmprp=enc%3AAQAJAAAA4HoV3kP08IDx%2BKZ9MfhVJKlidBnyf7d%2FGysvdEghs2dOzz%2FlD2s1nitWKFLEZCAiGoMGMeQ29%2BU4vTTujv54Mzb4ssD%2F8TGiEMhOPOeKaQHMaRSJw0bjbfsqc10ENgDq2GkLDdm62wxxrw%2BQ5zAMocLJwHBWCJsPDZAsZjdd6CIzq5mbAL3FktSdgIjmattz%2F6LdwT2DK875CEpsd5IZ6UuSs2BpAmG0MIuMNQoJ3--webV0ZrZ%2B7aiYGmWCg1KVwL%2Bv297nv%2BnrwvjJYO4ExG%2FNCAFDY%2FWebbu2pundlhjs%7Ctkp%3ABk9SR_jHgM2qZA