Here's the list of all available Mega evolutions...
Poliwrathite (Mega Poliwrath)
VenusauriteY (Mega Venusaur Y)
VenusauriteX (Mega Venusaur X)
Pidgeotite X (Mega Pidgeot X)
NidokingiteX (Mega Nidoking) (Don't get confused, there isn't a Mega Nidoking Y, at least not yet)
Politoedite (Mega Politoed)
SteelixiteX (Mega Steelix X) (The hack's original Mega Steelix)
Parasectite (Mega Parasect)
RaticateX (Mega Raticate X) (Should be RaticatiteX imo)
GolemiteX (Mega Golem X)
BlastoisniteX (Mega Blastoise X)
BlastoisniteY (Mega Blastoise Y)
CharizarditeX (Mega Charizard X)
CharizarditeY (Mega Charizard Y)
PikachuniteX (Mega Pikachu) (Once again, there isn't a Mega Pikachu Y, at least not yet)
MewtwoniteX (Mega Mewtwo X)
MewtwoniteY (Mega Mewtwo Y)
Alakazite (Mega Alakazam)
Gengarite (Mega Gengar)
Kangaskite (Mega Kangaskhan)
Gyaradosite (Mega Gyarados) (You can get this one in the Magikarp's Evolution Trial Chapter which is almost at the Orange's League, btw)
Aerodactite (Mega Aerodactyl)
Ampharosite (Mega Ampharos)
Scizorite (Mega Scizor)
Heracronite (Mega Heracross)
Houndoomite (Mega Houndoom)
Tyranitarite (Mega Tyranitar)
Blazikenite (Mega Blaziken) (Don't know where Torchic is tho :c)
Gardevoirite (Mega Gardevoir)
Mawilite (Mega Mawile)
Aggronite (Mega Aggron)
Medichamite (Mega Medicham)
Absolite (Mega Absol)
Garchompite (Mega Garchomp)
Lucarionite (Mega Lucario)
Abomasite (Mega Abomasnow)
Beedrilite (Mega Beedrill)
Butterfrite (Mega Butterfree)
PidgeotiteY (Mega Pidgeot Y) (The official Mega Pidgeot)
Slowbronite (Mega Slowbro)
SteelixiteY (Mega Steelix Y) (The official Mega Steelix)
Sceptilite (Mega Sceptile)
Swampertite (Mega Swampert)
Sablenite (Mega Sableye)
Sharpedonite (Mega Sharpedo)
Cameruptite (Mega Camerupt)
Altarianite (Mega Altaria)
Glalitite (Mega Glalie)
Salamencite (Mega Salamence)
Flygonite (Mega Flygon)
Metagrossite (Mega Metagross)
Latiasite (Mega Latias)
Lopunnite (Mega Lopunny)
Galladite (Mega Gallade)
Latiosite (Mega Latios)
Mega Banette (Banetite?),
There are 56 mega evolutions in total. And you can find some Mega stones in Valencia Island's Mart. Pumello island's mart offers all the stones available. Also you can find VenusauriteY and PidgeotiteX in Silph Co. And finally, you can get 5-6 mega stones through pickup ability...