Malcolm,
I just checked briefly at the RAB's website (www.rab.com) - under "Creative" -- "MP3" -- and selected the category "Radio". Up popped over 50 pro-radio spots, including classics by Stan Freberg, Ben Stein, Dick Orkin, and the "Heard It On the Radio" series. Lots of good stuff.
That said, access to these materials is limited to members. It's a great resource for radio folks, considering the wealth of materials available at RAB.