The IMSSA is currently spearheading a campaign to increase access to teaching assistants for IMS graduate students. Check their TA webpage to view any postings they have collected.
Some, but not all, Teaching Assistant positions at the University of Toronto are posted on the CUPE3902 website. The Undergraduate Human Biology Program also maintains a TA website
CUPE3902 is the teaching assistants union at UofT; their website provides additional information on terms and conditions of employment, benefits, pay rates, etc.