Distributing a .box with Windows would be illegal. Public / free boxes however can be found on the site Vagrantbox.es
You could try and build your own box with veewee. The author of veewee shows in this video how he built a Windows7 box: veewee does windows7, so Windows Server 2k8 might be possible too, since there are templates available from within veewee:
# veewee templates
... snip ...
vagrant basebox define '<boxname>' 'windows-2008R1-serverstandard-amd64'
vagrant basebox define '<boxname>' 'windows-2008R2-amd64'
vagrant basebox define '<boxname>' 'windows-2008R2-serverstandard-amd64'
You'll also need a W2k8 DVD or ISO-image of course.