From the very first day I started reading these boards, people have recommended two safe discount ticket sites: Mapleleaftickets.com and undercovertourist.com.
I have purchased from both of these sites - one has free shipping (perhaps both, can't remember) and they are both SO helpful if you need to call or have questions.
Anything else I've seen (similiar to the billboards are those popup sites that say 3 Day Parkhopper, $50 a piece) has a time share presentation that is necessary, and a few other things. Don't even bother. And you can try eBay - we did and were lucky to find an honest seller...but remember, people will say ANYTHING to get rid of tickets they're not able to use (ie expired, children's tickets, etc.). Good luck!