I have a shiny gardevoir in sun and moon but I can't mega evolve it. I know about the battle tree BUT ITS NOT THERE I NEED HELP
If you enter the tiny tourney and complete 3 battles you get Gardevorite, Galladite, and Lopunnite.
The gardevoirite is currently unobtainable in SM. It'll likely be distributed in an event sometime in the next few months.
list of currently unobtainable mega evolutions