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.
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.
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.