Where can i dispose of my 1 pound propane gas cylinders for no cost?

I live in Ontario Canada and own a family Campground near the town of Fergus. Over the past campers have left these one pound propane bottles in which we have to collect when they leave. I now have hundreds of these propane canisters and do not know what to do with them. Is there any place where i can take them and not get a huge bill??