I wonder if Freightliner may have them as an alternative to the rubber ones? If so, you can order Freightliner parts direct from the factory if they do. Give Freightliner Custom Chassis a call at their customer service number and see if they can help. They will at least give you the part numbers.
By the way, I've had poor results ordering parts thru Freightliner dealers (slow, wrong part, etc.), but when I call factory service the results are outstanding. Quick service, correct part and competitive pricing.