This app was mentioned in 2 comments, with an average of 2.50 upvotes
Zedge has a nice selection. I've used them before.
Now I just cut my own and use them. They're usually just video game/movie quotes I set for different contacts.
If anyone is curious, I use this site to rip the audio from the YouTube video and then use Ringtone Maker - MP3 Cutter to cut it down and save it. I've probably made over 300+ so far.