Here's 2007 info on public travel options from Pearson Airport (YYZ) Toronto to Belleville, Ontario: 

Coach Canada runs three buses a day from the airport to the Belleville bus terminal. On their link, make sure to select "Toronto Airport" (under Ontario) and not "Toronto". Fares are about $50 per person. Ontario Coachway runs door-to-door shared van service; $80 for one person or $110 (total) for two. But the the Via Rail train from downtown is actually the cheapest, by a bit. There are 5 trains daily, and if you book early enough you can get a fare of $27). To get to Union Station from the airport, hop on the Toronto Airport Express bus  for $16.95 per person. People can also take a Toronto Airport Limousine or Toronto Taxi from Pearson Airport if they don't want to wait for the bus or worry about the hassle of luggage carrying. The prices may be more than public transit but they get you to Bellville in express time.