I don't even use a website. If you have an SD card, that's the best way. Make them yourself -- this way, it's totally free and you can take whatever part of the song you want and make it however long you want.
Download Audacity (http://download.cnet.com/Audacity/3000-2170_4-10058117.html?tag=mncol). Make sure the song is in MP3 format and open it. Using the cursor tool (I usually zoom in a bit first), highlight over the area of the song you want, and hit CTRL+C to copy it (the song has to be stopped to do this). Then, open a new window of Audacity and hit CTRL+P to paste it. I'd play it to make sure it's what you want -- you can edit out anything that you don't want. When you are finished, go to File > Export as mp3. The first time you do this, it'll tell you that you need to download the LAME file -- go ahead and do that. You then have yourself a custom ringtone!
It's a little bit of work, but this is how I do it and I like it way more than any ringtones I've found. My friends have started doing it this way too. I also use the fade in/out features at the beginning and end of the tones so they don't start out loud and abruptly.
Otherwise, I guess you can use myxer.com. That's the only one I've ever tried to use, but you have to have the ability to get to the internet using via message.
waptrick.com its mobile only though you cant acces it on your computer or mac but they have every thing from Wallpapers,Games,Ringtones,and Full Length Movies no regestration or signing up required