Got any of those Lionel, Flyer or Marx billboards that need replacement, or you're just tired of looking at the same ones? Here's a great site that has many if not all of the originals, plus a few custom versions as well:http://users.zoominternet.net/~rrbillboards/Lionel.htm
They print in the appropriate size for mounting. I usually print them on my ink-jet printer, then glue them on to a bit of cardboard from a cereal or gift box and slide them into the holder. The Flyer ones can be glued onto your whistling billboards, replacing the faded originals (only as long as you're not a collector-purist, egads)...
You can also use them stand-alone...paste them onto the side of a building or make your own frame.
Here's one I printed a few months ago to replace the LCCA advertisement it came with:
Fun little project.