Vimeo.com offers originally uploaded files from pretty much any camera, so they are high quality and the best sample files to work with. You need to do the following.
1. Create a Vimeo account, it's free.
2. Search for a clip you like, it's best to hunt for videos that the uploader said that they are "directly out of the camera". That means .mts files out of AVCHD cameras, .m2t files out of HDV cameras, .mov files out of most recent digicams.
3. Be very careful to only download files from people who have a PLUS icon next to their username. See, Vimeo only offers the originally uploaded (higher quality) version of the file only for users who have paid for the Vimeo PLUS service. For all the other users Vimeo offers only the re-encoded-by-vimeo MP4 file, which is lower quality.

Here's such an example:
http://vimeo.com/9243986
The user is a PLUS one, and it's a direct upload from his camera, so the downloading file on the right sidebar is an .mts one (if it was Vimeo's re-encoded lower quality version it would have been an MP4).
And another one:
http://vimeo.com/25852187