You can get Alpha 1 here - http://www.mozillamessaging.com/en-US/thunderbird/early_releases/
Otherwise you can get the latest nightly build here -
http://ftp.mozilla.org/pub/mozilla.org/thunderbird/nightly/latest-trunk/
Please read the notice on the page:
"These releases are not for production use, but for developers and
testers to give us early feedback. To emphasize this point, these
alpha releases are called Shredder -- they do not represent the final
state of any Thunderbird release, can contain significant bugs, and
should be handled with appropriate care."
Josh