Surprisingly, Loblaws sells a good quality semi-sweet dark in a large PC-brand bar for about $5. It's French and works very well in baking in my experience. They might be little tough to find in July but it's worth a look.
Pusateri's at Avenue and Lawrence carries most of the quality chocolate (callebaut, sharfenberger, etc). Closer to you on Yonge is the small Dufflet's store, which usually carries some baking chocolate.
I have actually been buying large packs of bars of very good dark baking chocolate over at Soma. Excellent quality and all things considered not insanely expensive. The packs contain about 4 bars of about 7 oz each.