in UK - most costuming shops will have a fake leather trench to go with their generic WWII german uniform if thats all u are lookin for?
I presume, as usual, in the US it will just be all that much easier to find!
http://www.flagshipfancydress.co.uk/german-soldierofficer-coat-550-p.asp
a bit fake lookin but ok if its for a party or halloween - that sorta thing. I am even consodering one for a budget kroenen cossie