Yes most should allow you to use existing tunes however some do not
You will reply to the number that you receive and say no. So the word no means that, you didnt receive nor have it download
The first place to look for ringtones is your own wireless carrier, contact them Many of the ringtone sites are based outside the U.S, most of them will NOT work with your phone and/or wireless...
It depends on your phone but usually its under the main tab "Media Center" and the tab "Music and Tones"
Try GrooveShark.com