While people were waiting for "Before Midnight" to be released last year, the HD versions of "Before Sunrise" and "Before Sunset" were added to the iTunes online library. Two WEB-DL releases surfaced immediately after that and they are still easy to find today:
I've been looking for the 1080p version of the second movie for a while now, kinda hard to find. It surfaced on some obscure sites and I managed to download it. Be sure to watch all three movies in order and you'll be amazed by a wonderful story. Enjoy!
LE: The movie is now available on Blu-ray. Download links have been removed.