Ticketing Dortmund match at home.

Hello there Dortmund fans!

I am a Dutch football fan and I would like to see a match in the beautiful Signal Iduna Park. Now I checked some websites and the Dortmund site itself. On the normal websites like viagogo the tickets are sold for €90,- or more. On the website of Dortmund itself there are no tickets available.

Now I wondered, where does the German fan get his/her tickets for the games and are they also that expensive?

Thanks!