There are three main ways to buy a German low emission zone sticker (Umweltplakette).
1. German LEZ stickers can be purchased online for all vehicles in all countries for 17.50€ (from inside of Germany 9,90), also available to purchase in different languages.
2. Within Germany, they are available from every TÜV station. So for example, they can be bought in TÜV offices in towns without an LEZ without entering the LEZ. TÜV sud locations can be found by the search on their homepage. TÜV Nord offices can be found here, offices outside Germany where stickers can be bought are listed below.
3. You can buy German stickers in person in the TÜV offices in the countries below. Czech Republic, Sweden, The Netherlands, Finland. You can also buy them in Belgium from VAB Belgium office (VAB Pastor Coplaan 100 2070 Zwijndrecht).
Make sure that you leave enough time for the sticker to arrive! At least 6 weeks is recommended.
! Please beware, some internet sites charge you much more than the websites listed above!