There are tons of ways to get from Toronto's Pearson Airport to the heart of TIFF but the cheapest is via the Toronto Transit Commission (TTC). It takes about 90 minutes and only costs $3 Canadian. Here's a handy guide that will get you from the airport to within steps of Bell Lightbox.
1) After you leave your gate at Pearson go to terminal 3 and follow these signs:
2) Once you get outside go stand by the TTC pole:
4) The bus will make 1 stop before it reaches your stop, Kipling station, which is the end of the Southbound line before the bus returns to the airport. Go inside the station and get on the train. This station is at the end of the Bloor-Danforth line so no matter what train you get on you will be heading in the right direction. The Bloor-Danforth line is the green line in the map below.
5) After 14 stops you will arrive at the Spadina station. Don't worry about counting stops, just listen to the conductor as they announce each stop as the train pulls into the station. There are also maps posted in the train so you can figure out your location pretty quicky. At the Spadina station you are going to get off the Bloor-Danforth train and follow the signs at the station to the Yonge-University-Spadina line heading towards Finch. Do not go to the platform that says Yonge-University-Spadina heading towards Downsview as those trains are going in the opposite direction of Lighbox. Yonge-University-Spadina is the yellow line above.
6) If the first stop you come to is St. George you are on the right train. If you arrive at Dupont, you went the wrong way. Don't worry, just get off the train an wait for the next train going in the other direction (trains come every 10-15 minutes). From St. George it is 6 stops before you arrive at your final destination of St. Andrew which is a short walk to the Festival Box Office.