Look for "Sharon Gun Club". It is about an hour north of Toronto area (just a bit east of Newmarket). A wonderful outdoor club, I think there are 7 ranges there, from 25m pistol to 300m rifle.
In addition to Sharon, if you are interested in longer range rifles, look up "Ontario Rifle Association". They use several military ranges (Cedar Springs, Winona, and principally Borden), and conduct a long range shooting program from 300y to 1000y. They hold intro days, etc.