The Plaza Premium website shows the T1 International lounge does have shower access:
https://www.plaza-network.com/locati...l?city=Toronto

If that doesn't work, there is a workout gym called Goodlife in the pre-security area of T1 that offers $15 day passes including access to showers and change rooms. If coming out of international arrivals in T1, turn right and you will see it at the end of the terminal building.

https://www.torontopearson.com/Compo...d=4294986843##