Yes, MDM carries Sunshine (check the Sun Gro link) and a number of other brands of seed starting and other mixes. They cater mostly to the trades but also sell to others. I'd call first, though; not sure if it's seasonal for them.

Here is a link that might be useful: MDM

try JR Johnson near Rosedale Mall. I think their web site
is Hort Hippo. They have an extensive supply of the
Sunshine mixes as well as other supplies. I think they
supply the nurseries around here.
Lynn