PAX Pokemon League

Aside form the Mega Stones found in game you can get a bunch with mystery gift codes. Code INTIMIDATE gets you Beedrillite, Mawilite, Audinite and Medichamite. Code AZUL gets you Pidgeotite, Steelixite, Heracronite and Houndoominite. Code MATSUBUSA gets you Sceptilite, Blazikenite, Swampertite, Banettite and Cameruptite. Code SABLEVOLANT gets you Tyranitarite, Abomasite, Manectite and Aggronite. Code POYONG gets you Gardevoirite, Galladite, Lopunnite and Diancite. Can you only use the Mega Stones found in game or are mystery gift ones ok to use too?

Generally, the rule is that if it's available through any legit means (i.e. not hacking), it's ok to use. However, you'll want to check out the rules page for specific exceptions. Each PPL goes over the rules and decides if anything needs to be changed and the rules page is updated if needed. For reference, for PPL East 2017, these Mega Stones were not allowed: Kangaskhanite, Blazikenite, Gengarite, Latiasite, Latiosite, Mewtwonite X, Mewtwonite Y, Mawilite, Lucarionite, Diancite, and Salamencite.

You can find all of the rules here: http://paxpokemonleague.net/rules/