There are quite afew sites that you can get windows betas from, i used to have quite afew of them in my favourites but they have since been deleted (including 2 that you can download the Longhorn Beta from.
If you find a site that provides those betas please understand that such sites are illegal and (VERY RARELY) you may be prosecuted for using the illegal content that sites like that provide.
But if you REALLY want an illegal longhron, then i remember that the url was something like:
www.windowslonghorn.com
or
www.windows-longhorn-beta.com
something like the above, but i dont remember it at all.