Here's one of my favorite sites for billboards, thought you guys might like it too! Most of them are shown at an angle shot, so not really usable for our layouts, but many of them are straight-on shots and can be easily resized and printed out on your computer for your layouts. Easy way to add some interest or date the layout.
http://www.billboardsofthepast.com/index.html
http://www.billboardsofthepast.com/index.html