El Paso Saddlery makes an authentic flap holster for the Luger (based on the US Test Trial Luger holster design)
http://www.epsaddlery.com/pc-105-14-1900-holster.aspx
Originally intended for the Model 1900, it works just as well for other models and can be ordered from EPS in various barrel lengths and with or without lining. A bit on the expensive side but excellent quality...should last for as long as you need it.