Hmmm - I have not seen any TTC agents in any of the terminals (not to say they are not there). You are correct you cannot purchase on the transit bus itself. You may wish to pay the single to get to Islington station and then get your pass there. Or if you tell someone when you are coming in, possibly someone in YYZ can purchase and send you the day pass in advance? Depends I guess on when you travel.

SimonC