How to find the location of an icon file on OS X?
I couldn't find that exact icon because I wasn't able to produce such a certificate file. If you can tell me where to find such a file, I might have better luck.

I was, however able to find a similar image. It's located at /System/Library/Frameworks/SecurityInterface.framework/Versions/A/Resources/CertLargeStd.png (at least for Mac OS 10.8.2) and it looks like this:

(CertLargeStd@2x.png is the one used on Retina screens.)

This is the image shown when you view a certificate in Keychain Access or Safari:


To find this image, I opened up the Safari panel above. I examined Safari's open files via Activity Monitor, and the image in question was there.