If you want to blow through Pokemon Sun or Moon so you can catch all those Legendary Pokemon trapped behind the wall of a completed game, you probably won't have time to look for all the Pokemon in the games.
Luckily, some kind soul on Reddit has uploaded the QR Codes for a majority of the Pokemon in the game! These QR Codes won't magically give you the scanned Pokemon but it will show you where to catch them.
Scanning the QR Code will mark the Pokemon as 'seen' and show where it can be located on your map.
This is how it works, you can scan up to 10 QR Codes a day and get one attempt to catch the Pokemon you've scanned. With the QR Code scanned, you can run an 'Island Scan' and find the Pokemon that are rare to that island and can even spawn them.
Scanning a shiny does not guarantee a shiny.