Düsseldorf International Airport has a shop, if I remember right. I don´t know about Frankfurt and Munich.
But you can find several T-Punkte (they call their shops T-Punkt) in every city:
http://suche.t-punkt.de/tpunkte/
Just enter your city into the search form or select from the drop down list.
T-Mobile and
Vodafone are the best providers because they are available all over Germany. The others may have connection problems here and there.
Blau sells cheap cards and you can select your phone number, but it´s the E-Plus net.
You´ll find Vodafone shops in every city as well.