Sky Zero,
I found one on this very forum. Checking the For Sale section regularly is always good advice. This would hold true for the other Luger forum as well. I've also seen several on Gun Broker. Don't just search P08 Luger; you'll get more results if you also run searches for "DWM Luger" and "Erfurt Luger." There's always Simpson Ltd. as well,
http://www.simpsonltd.com/index.php. Gun shops and gun shows may be fruitful too, but I've found the above options to be more likely to have what you're looking for on a regular basis.