Hi mowmart,
Welcome to Unimog club uk,
You should be able to get the arch's from any Unimog dealer, or ebay.de
You can go for a used pair of originals, if you can find a good set, or you can get schuster ones, they're an after market thing, haven't quite got the roll on the edge like mercs, but they're a very good imitation and you wouldn't know without running your finger under the edge

Cheers, Brian