Ok, let me think. A good thing to try would be the Landesreitschulen or Landesreit- und Fahrschulen, e.g. the "Landesreit- und Fahrschule in Hoya" (close to Hannover) personally but I know that they have a great instructor Herrn Meyer zu Strohen. You can look it up on http://www.landesreitschule-hoya.de/lehrgang.htm where you actually find the data when courses take place. If you google the term "landesreitschule" you get about 8 to ten different names.
Another good thing to try are the state stud farms such as the Haupt- und Landgestüt Neustadt/ Dosse (they have nice horses there, mostly even stallions, I've been ridden there once on a student riders competition)(close to Berlin, East-Germany), the Haupt- und Landgestüt Marbach (Southern Germany), or the Haupt- und Landgestüt Moritzburg (close to Leipzig, East-Germany).
And of course you could try Warendorf, where the "Bereiterprüfungen" take place. They have quite a number of well-trained horses.
Just look around on the Internet and if you've found sómething that you like just contact them. Hoya and Neustadt/ Dosse are probably the best to try.
Of course you could try some private stables, too, but they normally take really a lot of money. In the Netherlands some of the really good riders offer these kind of lessons as well (e.g. Anky van Grunsven or Academy Schellekens-Bartels), they are more advanced in these kind of marketing activities but I belief you don't want to pay around 500-700€ for two lessons, do you?
I hope I could be of some sort of help. Just contact me, if you need more infos, ok?