What kind of thing are you looking for? Price range?
You're a wee bit early as many places won't have summer stock in yet though.
Apart from the classic Sonnenstr. wedding stores, here are 2 other ideas:
Nicowa has quite dressy things by German standards, stores on Hohenzollernstr. and Sendlingerstr.
Ludwig Beck also worth a go in their evening wear dept.
Mind you, does the wedding has a specific colour scheme?
Happy shopping!