Unfortunately, there is no way to get your tax back if you book with a discounter like hotels.com. You'll need the itemized tax on a receipt issued by a Canadian company with a proper GST number.

Booking with a discounter, they will never want to disclose to you the rate they negotiated with the hotel directly by issuing you an itemized receipt. Even if they do, they've no GST number. So likely you won't get a refund.