First off, check if the theme is available in wordpress.org themes: http://wordpress.org/extend/themes/
If it’s not, find the name of the theme developer and contact them. Many developers of wordpress.com themes are happy for people to use their themes on self-hosted WordPress blogs. The developer’s name and contact details can be found by looking at style.css. You can see the content of style.css by viewing the CSS file in your browser. Alternatively, navigate to style.css for the theme you want in the wordpress.com theme repository: https://wpcom-themes.svn.automattic.com/ This is also where you download the theme.
You must seek permission from the theme’s author before you download and use their theme unless the theme’s style.css includes a note to say it is released under the GPL. If it’s GPL then permission has already been given.
For plugins, you need to download them from wordpress.org. If they are not available there you will need to find acceptable replacements or track down who developed them and ask if the plugins are going to be made available.