AFAIK Mercedes no longer sell the rack (which I think was a Thule product) because it's no longer a recommended fit for the electric tailgate. They do appear second hand from time to time - keep an eye on the various forums, eBay, FB pages and Marketplace. The alternative is a Fiamma rack for the V Class/Vito/Viano - I bought mine little used from a Viano owner and it works well. Compared to the Mercedes product, it's also a good bit cheaper.
HTH