Skandaltracht is a great shop across from the Kreisverwaltungsreferat (KVR) by Poccistraße. It's a tiny little "hole in the wall" kind of place, but dead easy to find because it's right opposite the KVR - and everyone knows where that is, right?
They have Lederhosen as far as the eye can see, in all lengths and styles. The service is good with plenty of employees to help you. Because it's so small Skandaltracht doesn't have any changerooms to try stuff on, which supposedly only adds to the Lederhosen buying experience.
The prices are very reasonable. You can spend €200 and get everything but shoes - Lederhosen €150, shirt €30, belt €20, and if you're lucky they'll throw in the socks for free. All of it brand new, not second hand. They also have quite a range of Dirndls and other ladieswear. Although the smaller ones look like the kind of thing Madonna would wear.
Skandaltracht
Ruppertstrasse 32
Munich Isarvorstadt
...near Poccistraße U-Bahn
See also: Dirndl shops