You can get them from any source - and there are 1,000's of them for FREE - no need to pay for Ringtones. You can also creat your own Ringtone from a snippet of any MP3 or Wav file. Several free apps in market will do this. Just do a Search for "RINGTONES" in Market. One that comes to mind are RingDroids, But again, there are TONS of them ..
Phonezoo.com is another place to get them, too. They have a ton of them there - from songs to clips from movies & TV shows. Can even get wallpapers there, too. But I agree with the previous poster, if you want specific snippets of specific songs, RingDroid is the best! I had another app for making ring-tones, but as soon as I discovered RingDroid, the other got promptly deleted!