I am driving to Germany today and just noticed that I am traveling to a city which requires a so called "Umwelt-plakette" or environmental badge to enter. Normally you need to order them in advance. Is it possible to buy them somewhere while on the road?
According to Wikipedia you can buy it at various certification agencies (TÜV, DEKRA) and theoretically any auto repair shop that's certified for doing emission tests (though apparently many don't offer this service). You need to show them the vehicle license and registration papers, from which they determine whether your car can get such a badge, and which one.
Here's detailed information in English from a TÜV website, and here are lists of branch offices where you should definitely be able to get the badge: