Mike Wrona is a violin maker located just across the border in Lewiston, NY. He has a large assortment of instruments, including ones appropriate for students, and also performs repairs and maintenance.
The Sound Post in Toronto carries all string instrument supplies, a huge amount of sheet music, and offers fast shipping at a reasonable price. There are also many instruments for sale, in all members of the string family.
For general music supplies such as tuners or music stands, Long & McQuade is a convenient local resource.
Good selection, fast and cheap shipping, for very popular books such as Suzuki Violin or Suzuki Piano
Great for more specialized classical music, with a variety of different editions for different pieces.