alright you asked the question and wanted a post game spoiler alert, well, basically your first question only happens post game so
1) Spoilers: you can only get megastones for a few pokemon before facing the E4 for the first time, some can be obtained after that, but the bulk of them are only found after receiving an upgrade to the Mega Ring post game and then they can only be found between 8:00 and 9:00 PM, however itemfinder is not needed in order to locate them
If it's worth doing, it's worth over doing! There is no kill like Overkill.
after you beat the game you need to go to battle mansion in Kiloude, and win 1 fight, then fight your rival (same town, and they appear there daily for a battle) after you beat them they tell you the professor wanted to talk to you in Anistar City go talk to professor and your mega evolution doohickey gets upgraded after this from 8-9pm REAL TIME you will see little shiny spots those are mega evolution stones