I don't know of a script to do that. I guess you might serve your page from a piece of CGI and, using the the 'HTTP_REFERER' var, check to see if the user is coming from one of your sponsors. If they are not, then give them a page that tells them to visit a sponsor. If they are coming from a sponsor, let get to the games. Note that the HTTP_REFERER is not dependably populated. You could play around with a little to see if it would do what you need.
'just my humble opinion .... but, I would be out there quick after getting rail roaded like that.
Good Luck.
keep the rudder amid ship and beware the odd typo