You're in luck! It is not too late to grow the following from seed:
Summer Squash (zucchini, yellow straightneck, scallop, etc)
Snap beans (the bush variety)
Dry beans for soups, casseroles, etc (the bush variety)
Kale, lettuce, Swiss Chard, collards
Beets
Carrots (if you plant soon!)
Peas (bush/dwarf - if you plant soon!)
Cucumber (if you plant soon!)
I garden in zone 2, so if these things will grow from seed for me up here, I am pretty sure they will do well for you in Ontario. Planting transplants gives you produce more quickly, but if you can't find free transplants, free seeds might be the next best option. ;)