Actually for TV and radio there are several companies doing remnant ad sales which are fairly affordable and can be spun up pretty quickly. The hard part of TV ads on the fly is you need to have some production quality which can add expense and time. Not so much for radio. With billboards, I do know there are remnant sales there, but again you have a lead time to produce the banner that is attached to the billboard.
One company that comes to mind off the top of my head in the radio/TV space is Bid4Spots.com. Basically like Google AdWords for TV. There was talk at one time of Google offering a similar product, but I don't know what ever happened to it. Finally, there was another company that had an entire ad wizard for TV ads. Essentially you could pick choose music, or do a voice over, add some text, and have a generic video playing in the background. You could essentially create an entire ad and have it on TV in a day. I can't remember the name of the company though.