There are several options to add sound files as ringtones..
You can select any Mp3 file as a ringtone
Do a Google search for Ringtones.mp3
App market App
Zedge allows you to search for ringtones..
Go to most android forums and search for ringtones. I found the entire Droid2 R2D2 ringtones here
R2D2 Sounds? - Page 3 - Droid Forum - Verizon Droid & the Motorola Droid Forum
Download
Youtube Video converter Youtube Video converter download and convert any Youtube video to an Mp3 file than can then be loaded onto your phone.
Download free App
Ringdroid.. allows you to edit Mp3 files and make your own ringtones on your phone.
On your desktop use
Audacity to edit your own Mp3 files to ringtones.
Once you have selected ringtones to add be sure to place them on your SD card in the following directory.
sdcard/media/audio/ringtones
If your SD card does not have a media folder.
Create a media folder-
sdcard/media
within the media folder create an audio sub-folder-
sdcard/media/audio
within the
sdcard/media/audio then create the following 3 sub folders
ringtones
alarms
notifications.
Store any sound files you want your system to recognize as designated system sounds as follows..
media/audio/ringtones/
media/audio/notifications/, or
media/audio/alarms/,
Also by storing system sounds in proper directory (sdcard/media/audio/...) they wont show up in your media players' play-list.