I am not involved in volunteer services but I thought this might be useful to somebody, just as a possible source of ideas.
http://www.volunteerottawa.ca/vo-clean/index.php?/eng/about_us/mission_and_strength

I also know that one of my nieces did a turn as a volunteer on an ambulance service at Wawa when she was looking for experience to help her get into nursing.
I remember a few guys I worked with rigging who were volunteering for fire departments and other services to get High Angle Rescue certification.
So you might be able to attract volunteers who are looking for a way to break into a career. Maybe you will lose them once they are certified, maybe not, but it could still be a possibility.