I'm having a hard time with obtaining the post-game mega stones in Pokémon Y. I have beaten The Champ and rival many times & my ring has been upgraded. I have been looking daily between 8 and 9pm in their proper places. I think these stones appearing may be a one time deal unless the times are different in version Y.
So I found all the mega stones in my game through this video (Except for the X version exclusives). Easy to follow, just pause the video at some places so you can fly there to get it. Only hard ones to get is Alakazite and scizorite.
Also, If you didn't already know,
Houndoomite, Aggronite, and Heracronite are all found in version Y. Although the pokemon corresponding to these stones are found in version X.
Manectrite, Tyraitarite, and Pinsirite are all found in version X. Although the pokemon corresponding to these stones are found in version Y.
I wouldn't reccommend trading your version exclusive mega stones for the other ones, since you can only acquire them once in the game.