I remember seeing this at Dominion's when I was really young!
I would call supermarkets in neighbourhoods where these sandwiches might be more popular. Maybe Leaside, Rosedale...
Also, check out the smaller bakeries, as the previous poster suggested. Just grab a yellow pages and call the places that look they might be a good candidate - avoid Chinese and Italian bakeries and the like.
I recall my aunts and my grandmother having coloured bread at wedding and baby showers. I would imagine your best bet would be in a small private bakery. If you ask at the counter, I am sure they would put in a special order for you. Smaller places seem to be more likely to cater to special orders opposed to loblaws or costco.