You can order it for various countries from https://umwelt-plakette.de/sprachauswahl_foreignshop.php.
The site has also some details, e.g. for which cities do you need the sticker at all.
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:
Credit:stackoverflow.com‘
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