You could get them from an MB dealer (big money) but there's also several websites that do car spares from salvage that can get you genuine but used parts. A quick search on google should do the trick. The UK's not known as 'rip-off britain' for nothing. I wouldn't expect to get anything cheaper here than you can expect to get back home.
Euro Car Parts is the best bet as a non-dealer source, they have branches around London mostly accessible by the Tube....much cheaper than the Merc Dealers but ask for genuine parts as they sell pattern as well! Also, German-Swedish-French are a good source with various branches around London. Check the websites for both and you will have an idea of the price range, however, I would have thought that parts in the US are much cheaper? They are in Canada but they don't have the Vito/V Class in Canada for some strange reason!
Visiting London ~ Where does one buy genuine parts 
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