on amazon.com, the mp3 downloads, i did a search for ambient sleep sounds, ocean wave sounds, rain sounds, etc.
They have lots of different ones to choose from, most lasting over 70 minutes, most for .99 or .89. I downloaded three different ones, of rain and ocean waves, and burned them onto a cd.
Then when I get ready for bed, put the cd player on "repeat" mode and play it all night. It has really helped me keep asleep and stop waking up from nightmares.
They have ones specifically labeled for kids' sleep sounds too. I was told by several people to buy a "white sounds" machine. I looked up several different brands of those and none of them had real good reviews and they are expensive. But fixing up my own cd's and playing them on a regular cd player works great.