There have been a number of Mega Stones that have been missing from Pokémon Sun and Moon, and now there is an actual method to attain them. According to Serebii, the missing Mega Stones would be distributed during online competitions such as the upcoming 2017 International Challenge February competition. The Mawilite and Beedrillite stone codes will be distributed to players during this competition, where a limited number of codes will be given “ahead of a wait period of 1 to 2 months before all players get the code.”
You will also be able to get the missing Mega Stones through various events and online announcements with no wait time. The method will depend on the Mega Stone, and The Pokémon Company plans to use these methods through the summer.