The BBFAQ has a lot of good info.
How do I get ringtones on my BlackBerry? - BlackBerryFAQ
What kind of ringtones/audio can my Blackberry have? - BlackBerryFAQ
How to Convert Ringtones - BlackBerryFAQ
You can make your own ringtones here by uploading songs from your PC.
Make Your Own Ringtones - Free Ringtone Maker
The ringtones are located in device memory unless you have them on a Media Card. A media card is recommended because otherwise your device memory will fill up quickly.