I use a place called www.mobile17.com
. You just register, for free, and then you can use your own mp3's to make your own ringtone. It takes some time to send it to your phone since a lot of people use it (like an hour), so I will just make one and then let it send and it'll randomly show up on my phone. I think, since I have Sprint, it sends me to a website where I can just download it to my phone rather than getting a MMS message or whatnot. Myxer - Download Ringtones and More
is another place that you can create your own AND use some that have been pre-made. I know they have stuff like Family Guy and all kinds of movie and show ringtones as well...
Hope this helps!