I've never seen one in Ontario either, but then I've never been to NW Ontario! I will be up there this summer, though, to do some of the provinicial parks, and I hope to see one then.
Something I was told by a phD student studying red-sided garters, was that the ones in Ontario tend to not have very much red visible. In fact, she said that the easterns (which often have varying degrees of red or orange) often had more red than the 'red-sideds'. As is often the case, colour and pattern is not the most effective method of identification.