Seniors can sign up for the mass-immunization clinics at: https://covid19.ontariohealth.ca/
https://vaccineto.ca/ - list of hospitals/health-care clinics largely vaccinating seniors and high-risk populations
https://covid-19.ontario.ca/vaccine-locations - list of pharmacies giving out the AstraZeneca vaccine to 55+ year olds
Some pharmacies offering online registration:
Most likely will be vaccinated in a mass immunization clinic.
(last updated April 5, 2021 @ 12:11am)