I obtain audio recordings for my mp3 players from several locations. Some come from storybook CDs I already own, others have been purchased on Audible, but the cheapest (and most fun) source of materials for your listening center is to make your own!
I recently had SO much fun creating an audio recording of Doreen Cronin's wonderful book Click, Clack, Moo Cows that Type for my kinders. I was the voice of the narrator, but I enlisted some of my fellow staff members (including the school custodian) to provide voices for Farmer Brown, the cows, and duck.
This book had too many words for most of my students to follow along, so I added prompts each time they would have to turn the page. A boring old beep would simply not be fun enough for my kiddos so I stuck with the cow theme and added a MOO sound as the prompt. I also found a sound clip of a typewriter online and added this effect each time the story referenced someone typing a letter. As a final touch, I added a fun splash to the very end of the recording (when the ducks get a diving board). All of these sounds can be found online for free.
It's easy to get started. All you need is a computer with audio recording software and a USB microphone. The microphone doesn't have to be anything expensive. Mine is a headset made by Logitech. You can buy one Amazon for under $30. But any microphone with a USB connection will work.
Software is also easy to acquire.
I'm a Mac user and Apple provides a wonderful audio recording program in the iLife suite called GarageBand. I use this when I create audio materials for my classroom. It's pretty simple to navigate and use.
I know most people are PC users so I have good news for you as well. You can download Audacity for free. They have free downloads available for Windows, Mac, and Linux users.
Your students will think you are the coolest kid on the block if you make an audio recording just for them. Good luck and happy recording!