The 747 is my go to alternative if I can't get a lift to the airport. It leaves from the bus station at Berri and has multiple stops along Rene Levesque before heading to Lionel Groulx metro which is the last stop before the airport. It is a $10 dollar trip but if you have a day, 3 day, weekly or monthly pass it is included in those prices. The bus passes frequently too which is always a good thing and the one time I took it it wasn't crowded at all. There are racks to store your luggage and free wi-fi too!