Despite the fact that they carry and list a number of oddly engraved Walthers (and we've all laughed at some of their offerings in threads here in the past), I find myself regularly checking out the Collectors Firearms site; they have a good supply of vintage Walthers as well as some of the newer models -- and, like Simpson's, their stock tunrs over regularly:
I'd also urge you to visit the P38/PPK Forum and browse through the For Sale section there; every now and then, a good PPK turns up ... one that makes you wish you had some extra money burning a hole in your wallet:
Unless you have a LOT of hands-on time with MANY prewar and WWII PP-series pistols, and know exactly what you want and have the knowledge to evaluate exactly what you are getting, I advise patience rather than instant gratification.
Put the money secretly away for a while and go to some gunshows and gun shops and closely inspect as many different examples as you can find. There is a vast difference between prewar and WWII production. Education comes first, then purchase. If nothing else, you'll develop preferences and be able to discriminate among the many variations. If you are acquainted with a RKI who has some Walthers of this era, request some show and tell. At the present heart-stopping price level, it is very easy to get burned-- badly. Fortunately these models are not so scarce that you have to snatch the first opportunity that comes along.