Firstly, you need to get your Lumiose city style all the way up. The fastest way I've found to do this is buy premiere balls one at a time and get a Fufrou groomed. It took about 177 premiere balls to max my style. Having max style should drop the mega stones' price down to 10,000.
Secondly, getting money can move by pretty fast with the three-star restaurant, and you can train Pokemon while doing so. Just grab a high level with a multi-hit move like Heat Wave or Earthquake, give it either the amulet coin or luck incense, and place it in the center of Restaurant Le Wow's triple battle option (I use shiny Kyogre with Water Spout :D). Make sure you have the prize money O-Power going as well for maximum monies. To speed up the process even more, turn off battle animations in the menu. When you beat the restaurant, they'll give you Balm Mushrooms to sell, and the cash from those plus the Bargain O-Power should give you plenty to spend in no time.
Since you have Y version, there isn't an in-game way to get the X version Mega Stones. My only advice would be to change your status message in Passerby, ask on an online forum like Serebii, or trade from a friend. If I left anything out, forgive my absentmindedness, hope this helps!
