Just my own opinion here with nothin to back it up, but you don't find many Walthers because there are not many to find.
S&W has kept the lid on walther for many years. Can't really say I blame them; in my opinion walther makes a superior product. If you were the importer for a competitors product, would you really talk it up an take away from your sales? Take the shield for example: it's a pretty blatant PPS ripoff. If I was Walther I'd be awfully angry.
So, since sales are relatively low for walther (I think they are the least well known of all the major brands) not that many get imported. Think small demand results in small supply.
Now with the split from S&W hopefully walther can make a better headway into the US market. Walther does make one hell of a pistol.