This is a list of all the Mega Stones that can be obtained in Pokemon Sun and Moon. As you can see, all of the Mega Stones that you mentioned are not available in Gen VII (except for Charizardite Y, which can be obtained from defeating Red at the Battle Tree). This info can be backed up by Serebii, as they have updated their ItemDex pages for the Mega Stones and they, too, have yet to find the Manectite or the other Mega Stones in Sun and Moon.
So yeah. It's not available until further Gen VII instalments make it available, or an event distributes the Manectite.
Hope I helped. :)
Source: Above link and Serebii's ItemDex