HI. It really depends on what type of ticket you want/need. As previously posted, Mousesavers is the best place to determine what type of ticket you need. Also, you might want to look at where you are staying and if they offer shuttles to all the places you want to visit. A car might be needed. Alot of our guests use www.ticketmania.com . They are very reliable and can send your passes by mail ahead of your trip or you can pick them up while there (you will need a car for this). You do not want to buy them on ebay or similar sites as there have been many reports of problems with this.
If you have any other questions, I may be able to help.
Have fun!!