The only Mega Stones you are able to obtain before the Elite Four are:
You get latios. Then mega stones. BOOM POW ZAM kapow.
You find the Gyaradosite at Couriway Town. After having beaten the Elite 4 and done the side quest to allow you to find Mega Stones, go east to the area with waterfalls, where you'll find a shining patch. Go up to it and press A to get the Gyaradosite.
After you beat the Elite Four and Champion for the first time, you'll be told that Professor Sycamore is waiting for you at the Lumiose Station in North Boulevard. He will give you the TMV Pass which allows you to travel to a new place called Kiloude City. Here, you can find Calem/Serena but only after battling in the Battle Maison at least once. After winning againt your rival, you'll receive Absol's Mega Stone, Absolite, and he/she will tell you to go to the Anistar Sundial in Anistar City. Here you'll get your Mega Ring upgraded by Professor Sycamore. On a side note, certain Mega Stones are only obtainable after the Mega Ring is upgraded and can only be found each day between 8 pm (20h) and 9 pm (21h). You can check the location of these Mega Stones in the related questions below.
Not normally, but they can evolve into their Mega Evolution forms in battle through the use of their respective Mega Stones.
There is no such thing as a mega stone in Pokémon Pearl. Mega stones were first introduced in Pokémon X and Y and are required for certain Pokémon to Mega Evolve, which is a temporary new evolution stage.
You get latios. Then mega stones. BOOM POW ZAM kapow.
In Pokemon X and Y you are not allowed to sell mega stones unfortunately. They are considered a special item.
Mega City Four ended in 1996.
Mega City Four was created in 1987.
You find the Gyaradosite at Couriway Town. After having beaten the Elite 4 and done the side quest to allow you to find Mega Stones, go east to the area with waterfalls, where you'll find a shining patch. Go up to it and press A to get the Gyaradosite.
After you beat the Elite Four and Champion for the first time, you'll be told that Professor Sycamore is waiting for you at the Lumiose Station in North Boulevard. He will give you the TMV Pass which allows you to travel to a new place called Kiloude City. Here, you can find Calem/Serena but only after battling in the Battle Maison at least once. After winning againt your rival, you'll receive Absol's Mega Stone, Absolite, and he/she will tell you to go to the Anistar Sundial in Anistar City. Here you'll get your Mega Ring upgraded by Professor Sycamore. On a side note, certain Mega Stones are only obtainable after the Mega Ring is upgraded and can only be found each day between 8 pm (20h) and 9 pm (21h). You can check the location of these Mega Stones in the related questions below.
Not normally, but they can evolve into their Mega Evolution forms in battle through the use of their respective Mega Stones.
Yes Latios and Latias will have Mega Stones however they will be distributed when Omega Ruby and Sapphire come out in November.
There is no such thing as a mega stone in Pokémon Pearl. Mega stones were first introduced in Pokémon X and Y and are required for certain Pokémon to Mega Evolve, which is a temporary new evolution stage.
I think about the closest you'll get is the Covenant Combat Unit that has some Covenant Elite soldiers in it.
Both of these Mega Stones are found at the Cyllage Gym after beating the game.
Four Mega Bites