Artefacts have to be earned Commander. It would be boring if you could buy anything in the game.

its ok but you become only the same artefacts what could i do to become others?

It differs Commander and depends on the attack and losses.

Yep, the different types of artifacts drop from different size battles, with the rarer ones only coming from larger battles (although even the Unique rarity ones only require a few mil power of troops dying for a chance to drop).