The best site we have found to explore all aspects of outdoor advertising is that of the Outdoor Advertising Association of America, which is at:
http://www.oaaa.org/. You can see examples of PSA billboards, get design tips and other useful information.

To see templates and other information for specific types of outdoor advertising, go to: http://www.oaaa.org/OutofHomeAdvertising/OOHMediaFormats/OOHMediaFormats.aspx.