I have Grundig 500 that needs some work and spent some time looking for a place to have it repaired. I found out that apparently Grundig doesn't have factory service centers in the US anymore. My search did lead me to:
ACT ELECTRONICS, 429 Ash St., Henderson, NV 89015.
It's a place that specializes in Grundig repair. Perhaps they would have the part you need. Good luck.
Here's a link to their website:
Act Electronics
Terry