Doesn't exist. Not here, not at Logitech directly. We live in a throwaway society.

Here's what you can do:

a) use any suitable mouse mat (without a metal base, of course) such as https://www.digitec.ch/de/s1/product/corsair-gaming-mm300-m-mausmatte-5751544.
b) Contact Logitech within the warranty period. They will replace the whole thing, which is not very ecological.