Hi Simon, I imagine they are difficult to locate to be honest. My first efforts would be: eBay, Gumtree, specialist MB breakers etc etc and I think you'll be extremely lucky to find one in the correct colour, probably have to budget in to get it re-sprayed?
Its very much a search and explore situation. As mentioned pay attention to the usual concentrators & express your interest with the specialist breakers.
Many more pano panels available in USA.
Be prepared to pay much wonga and remember to check if the outer roof panel also comes with the inner headlining and a reasonable blind. Often they do not.
Also, again as already said, budget for a respray, even if in the right colour it may well be marked.
Good luck - you will find one eventually (I did)