Let's be honest, you can never have too many Mega Evolutions in Pokémon Omega Ruby and Alpha Sapphire. The only way for you to have too many is if you cheat and have a Pokémon that isn't supposed to exist and you Mega Evolve it.
If you're looking to get the Mega Stones for the Pokémon listed below, you might want to go ahead and make sure that you've already had the Primal Groudon and Kyogre event occur. If you have yet to experience the event play for a bit longer and then come back for the Mega Stones.
Without further ado, here's how to obtain the Mega Stones:
Galladite | Gallade
Galladite is not, by any means, hard to get. All you have to do is swing by Professor Cozmo's house in Fallarbor Town after the Delta Episode for Primal Groudon and Kyogre has occurred. Give him a chat and he will hand it over freely.
Garchompite | Garchomp
Aarune will give you this Mega Stone once you have captured 1,000 flags from secret bases (thus achieving Platinum Rank) in the Secret Base Guild.
Gardevoirite | Gardevoir
Wanda in Verdanturf Town will hand you Gardevoirite after the Delta Episode for Primal Groudon and Kyogre has occurred.