I posted the book Revolutionszeit im Bezirk Garmisch 1918/19 (German) by Josef Ostler (Mohr - Löwe - Raute) as the prime source on Freikorps Werdenfels in my little bibliography post on the subject about a year and a half ago.
Already at that point it was OOP and rather tricky to get for a decent price.
The user darkfire of the Sweetwater Forum just pointed out to me, that a book shop at Garmisch-Partenkirchen still has several copies of the book available. Apparently this shop was involved in the publishing in the first place.
Here's a link the shop: buchhandlung Adam
(If the link shouldn't take you directly to the book, just put the book's title given above into the 'Schnellsuche' (i.e. 'quick search' field.)
I can't comment on the shop's policy on shipping abroad or the shipping costs as I couldn't find that information on their homepage but it is surely worth dropping them an e-mail: buchhandlung.adam (at) t-online.de