I would try and come up with your own messages actually, stuff that means something to you, areas of your personality that you want to bring out more, or give more confidence to etc. I would think that you'd have more chance of success if the things you want to reinforce are things you feel strongly about in the first place, rather than random inspirational messages that you may not really be able to "latch on to," so to speak.

And yeah, you could just get a cheap computer mic if you don't already have one and record a few messages, then burn them onto a CD or stick them on an mp3 player or whatever floats your boat. I like the idea of the initial music that Zhaylin suggested, to help get you off to sleep first. Then you could just put your recorded file on the CD/playlist a few times in a row so it just plays through them as you sleep.

You can actually go and and buy stuff like this, but I don't see the point when it's not that difficult to rig something up yourself.

Good luck!