Geez, That gun is absolutely magnificent, however, those British proof houses sure know how to mangle a set of proof marks. The bottom of that Ejector rod housing looks like a graffiti wall.
I find them the same way that you find all of those beautiful Model 39s... That... AND I have been very lucky the last little while to have been the first engraved .357 Magnum collector to run across several beautiful Magnums. I also follow a daily regiment of checking all of the public auction sites and several of the public listing sites for engraved S&Ws. In addition, I do a general google search several times a month just to see if something pops up from some other source. Occasionally I get a referral from a fellow collector - those are always welcome.
Mostly, I count myself lucky. That is why I take the time to photograph the guns and share them here, so that every one who wants to can share in the beauty of engraved S&Ws. I ALWAYS enjoy it when others share as well. Ever since Kindergarten, my favorite subject was always "Show-N-Tell".