Megabus Canada, Megabus USA, and Greyhound Canada, all have service out BUF to Toronto but it is a long ride and there are only a couple of trips on each - and the prices vary. You might be able to book a through fare on Porter in its partnerships via Midway. If you do choose BUF, then consider a rental car and staying where you can park it for free. There are GO train stations on your way in where you could leave the car for your return.