Looking at the online seating it appears that it will be packed in EDM.
In Montreal it still shows tons of open seats. I suspect that is not accurate with season ticket holders seats not being caught up with the system. Considering that they had about 23K the last two home games a good shot this will be sold out IMHO.
Hamilton in the east may already be sold out.
With either EDM or SASK heading to Calgary that game as well should be very packed.
As soon as the two teams are identified that the Grey Cup will fill out the remaining seats quickly. More and More people make the annual trek to the Grey Cup for the festivities. Again when the teams are identified likely if your team is in it you will get tickets to the game.