I'd start by image searching "40's pin-up", find a couple images you like, and then track down the copyright holder (though some won't be copyrighted and some will be impossible to track down), it's a start!
The other thing I'd do is look at sites like Shutterstock & IStockPhoto as they're stuff (once purchased) can generally be used however you'd like (read the terms of use as it does vary somewhat).
There's also photographers sites (or sites for photographers galleries) that might turn up some interesting pics. Those sites would make contacting the copyright holder fairly easy.
I think for the most part you are safe, since Jeff is selling the Chuck and not the image...and he's not charging more for the application of the image. It's like buying window tint and asking that they also apply this sticker you found.
Assuming you weren't using said sticker to advertise or sell anything. If I use anything I find on deviant art, usually an message to the artist just saying "Hey can I print this on a decal to apply on something for my own personal use" is usually enough.
But if it's watermarked, I assume its not cool.
Thats my own twisted justification of it anyhow. I mean, I can take a picture of any logo, or print off any picture and apply it myself, I'm just having Jeff print it and clearcoat over it for me
