I'm JS. If you go to Samsung's website at www.samsung.com, click on "contact us" and they will transfer your call to their parts department. Make sure you have the phone model number (Telus Galaxy S3 White SGH-I747M) so they can match up the right back cover with it. It is better and cheaper to order parts through the manufacturer rather than second hand sites such as ebay or amazon as you could be waiting quite a while for the part to arrive in your mailbox possibly 4 to 6 weeks.
JS here again. Just found some more info for you. Try this website www.samsungcanadaparts.com or call toll free at 1-877-407-6593.
The service hours are EST: Monday to Friday: 7:30am to 8:00pm, Saturdays: 8:00am to 4:00pm, Sundays: CLOSED
$8.95 shipping across Canada. They also accept Visa and Mastercard.
Best of luck finding the back cover for your phone.