Um... Well, the place where I got a nice sheet of aluminum happened to be in Oakville... just off of Spears road. For you, that's literally a 45minute drive (Although you can walk to the place in 10minutes from the Oakville GO station)...
Like I always say .... your best bet is to check your local listings for welding or sheet metal shops in your area. If they cannot help you chances are they will gladly point you in the right direction if you ask. I'm a metal eater from way back & that is where all my materials come from.