I was on travel and left the last CD of an audio book I was listening to in the player.  I called but they can't find it.  I was wondering, is there a source to buy a single CD from audio books so I can replace it and then get it on my bookshelf to swap?  Thanks,

All I could imagine is contacting the publisher to ask.

Well one time my cassette player ate a Harry Potter tape.  I still had like 10 tapes to go.  I contacted the publisher (I think it was Listening Library) to ask if I could buy just that cassette.  They sent me a whole replacement set of tapes free of charge.  So it's worth a try.  If the # isn't on the box then try Googling them.  All they can say is no right? 

Awesome!  I never thought to contact the publisher. I'll give that  a try today.  Thanks so much!

I've had similar good luck with publishers - years ago I had to replace a cassette in a library book and they sent it for a nominal fee right away.