Keep going East till you see Valencia, the go south.There, you see Artix. Do his quests to earn badges. You can trade a badge to get the armor. Then click wear armor.
The only way to get a guardian armour in dragonfable is to have already bought the guardian membership for adventure quest then you just confirm it on the main dragonfable home page.
you cant buy armour you just have to train in that class like at necropolis you can train to become a paladin you have to get an undead slayer badge to get the armor then get more to unlock more...