I think I have only seen two advertised on here in 12 years.
Garvin and rockdrill on here might have an idea.
Shops to check would be City Air Weapons in Birmingham, Manchester Air Guns and Surrey Guns.
The best place to look for one would probably be egun.de and try to find someone who will ship outside of Germany.
I believe they were more popular in their home country where running target has a bigger following.
I don't know what the Running Target name is in German though!