I don't use vBulletin, rather phpBB, and out of experience have found that the best place to look for modications / themes is on the site of the developers, as they are more often than not supported, and you can get help.
A lot of bulletin board developers will not offer assistance unless you download a theme/mod from their own site.
Just my 2 cents on the matter, if you wish to search the internet for 3rd party themes that aren't officially suppored, that's fine, just be wary with them.