By the way, for those asking about the locations of mega evolutions (spoilers ahead): You start getting them after finishing the kanto saga. First you get the Venusaur stone in the Cave of Origins from Mr FUJI as part of a plot. As you continue the plot into the Orange islands, I don't exactly remember which town, but the one with the shelders that block your path - in one of the houses (not plot related, you just have to check the normal houses inside the cities) you will get a poliwrath stone. A city or two later (Moro Island), you will find a Pidgeotite you can pick up. It's not really hidden and hard to miss. The Raticate Stone and Golem stone will also appear later, but they require a bit more exploring. There are more stones, but anyway you can find the remaining mega stones (such as the Steelix stone or the other venusaur stone) on Mandarin Island gear shop after the Prince-Princess festival. You will encounter it before the 4th badge town.
Also, at some point during the kanto adventure, I'm not sure when/how it happened, but I found that my Meowth had a Swampertite equipped... not sure if it happened because of some event or because it held it when I captured it as a wild pokemon