If you are talking about the center insert on the 22.5" Accuride wheels, you can order then direct from Freightliner parts. I use factory direct parts ordering - my experience with Freightliner dealers parts ordering has not been very good.
You can call 1-800-FTL-HELP (have your VIN handy) and they will identify the specific part you need and give you a part number and phone contact for ordering the part. The call in your order. Quick and easy, and the prices are competitive, in my experience.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL