In eBrandon's classified section under Jobs, there is a separate category for volunteers.
https://ebrandon.ca/browseads.aspx?cat_id=110
You could also contact the post-secondary schools (BU and ACC) to see if they have a student volunteer group and/or if they could post something on their websites or in their student newsletters. Depending on the age groups you are looking for, you could also try the high schools as they often are looking for volunteer opportunities for students.
Shoppers' Mall lets volunteer organizations use a kiosk in the mall to educate people about the organization and you can also sign up there.
Burnout is definitely an issue and sometimes volunteers do need to take a break. Best of luck to you in finding some new blood.