I use Google. There’s a button at the top that says “Images.” Then I type in the product. For example, entering “Coppertone” will give you a bunch of images of the classic “little girl with the dog” ad. You may want to put “poster,” “ad” or “billboard” in the search, too.
This is one of my favorites:
I downloaded the image, sized it in MS Word, and printed it on decal paper. Then I applied it to the building, after painting a white rectangle over the space to get the colors right.