Hi from YYZ. There is indeed a mailbox in T1 across from aisle F, near the entry to US customs. I found stamps at the Macleans newsstand on the arrivals level. She charged me $2.25 for two stamps which seems a bit high, but I suppose beggars can't be choosers.

FYI, US post offices only sell official US post office stuff. Most Canadian post offices are contract stations in regular stores who can sell whatever they want.