So, every once in a while, new mega stones are released in the special stages. Lucario's mega passed and Blastoise's in currently happening. Say you don't make it in the top 20,000 to earn a mega stone. Is there any way to get another one later?
No, there isn't unless Pokemon decides to do another competition
Source: Well common sense and also Pokemon's official website because it said That there would be no way to get it after the event