There still is a secret source for coffee mugs. As I got much useful information on this site, I am willing to share the secret. One caffeine addict cannot let down fellow addicts after all.
These things are still readily available and can be used as coffee mug when push comes to shove:
Seidla (half quart stein)

These mugs are used in my region:
Häfala (small mug)

They keep the coffee warm and can hold a reasonable amount of coffee. As regional products they are not affected by international shortages.