Did you contact Blue Ox?
Etrailer seems to have them in stock:
https://www.etrailer.com/dept-pg-Accessories_and_Parts-sf-Weight_Distribution_Hitch-pt-L~Bracket_Plates.aspx
Worst case a good machine shop should be able to fabricate one without a lot of difficulty...probably won't be as cheap though.
Tammy & Mike
Ford F250 V10
2021 Gray Wolf
Gemini Catamaran 34'
Full Time spliting time between boat and RV