The problem is, the ones offered by my service are limited and I want to download one from the internet.
What’s the best way to do this? What are some reputable sites?
If you have a song in mind, just search Google for its name and “midi”.
http://www.myphonefiles.com/main.php
the best bargain on the net, you can make your own ringtones with existing mp3’s you have, download ring tones others have made AND you get UNLIMITED uploads to your phone for the measly price of 3$ a month.
I love it 
If you already have MP3 files and your phone support MP3 ringtones, you can use use any audio editor to clip and reencode them for your phone (30 seconds, 22 kHz, 32 kbps), then transfer them to your phone with a data cable or email.
A better bargain is Mobile17. It’s very similar, but it’s free, if you don’t mind waiting. (Usually about 30 minutes before they text you, and I think it’s accessible for about 24h after that) Or you can ‘donate’ which will give you credits that give you immediate access.