Check the Dollar shops. Here in Australia you can get nature sound CDs for about $2 each instead of $30 average for a CD. A good variety too and some have some light music which can make hour long listening more enjoyable. Good for variety. Also someone once posted a link for a program called Atmosphere Lite. You can record the sound effects like rain, streams and thunderstorms.
http://www.relaxingsoftware.com/atmlitehome.htm Alex
