Instead of getting them en-masse we can gradually work them into the story, just like the Armbands. First one or two, then another one or two but for another reason, then another one or two given a plot element of the moment, so and so, etc. That makes it much easier to justify and makes sure that we can get the ones we need, for the most part, instead of getting into a situation where for example we'd have 10 Lucarios but only 2 Lucarioites, or something similar.
There's no reason why we all have to get Mega Rings "right nao" (speaking in the timescale of Kalos arc), and Megas are not storyline-needed at all to my knowledge (and even if they were, we are not the PCs of the games, and our mons are good enough on their own).
Of course, as an alternatively and if it s true that we are going with Shadowified Megas for the finale, we can pin the mass availability of Rings on some random Cipher (or a splinter / double agent) plan for world domination.
“Thou hast forgotten the face of thy Lord. Remember, mortal, and fear pet me.”