I would use Google and search for "vBulletin 4 Styles" or similar. I will go ahead and express that you need to do your homework, don't just find a site with with a shop that has awesome looking styles for sale. Remember not to judge a book by it's cover, make sure to find out in their forums if members are currently happy, if they actually upgrade the styles promptly after a new version is released etc.
Alternatively there are some nice styles in the "Styles" forum here so be sure to check those out:
https://vborg.vbsupport.ru/forumdisplay.php?f=247
Good Luck!