If you're visiting Canada with a GSM unlocked iPhone 5, you have several Pay As You Go, or prepaid, options to consider. Nano-SIM availability is still hit and miss. There's not reason you shouldn't be able to get one, since every major carrier and subsidiary is currently carrying the iPhone 5. However, getting them to give you one for short-term PAYG might be a battle, or could incur an additional charge.
Canadian carriers like Rogers, Bell, and TELUS all have discount subsidiaries, namely Fide/Chatr, Virgin, and Koodo, so we're going to focus on those. Other carriers, like Moblicity, Wind, and Videotron use AWS frequencies (like T-Mobile in the U.S.) and don't currently support the iPhone.