I do not think there is a charter
company in Orillia, however you can check Quite Waters Sailboats, Canadian Yacht Charters, Cosmos Yacht Charters, and Discovery Yacht Charters.
Another option would be to visit the waterfront and see what is available there. I am not familiar with the sailing community there. It is hard to visit Orillia from Lake Simcoe due to a very low bridge at the narrows.
Hope this helps you a bit. May I ask what is your involvement with the Mariposa Festival?