Disassembling is no longer a profession of its own. It was merged with Leatherworking. You can't salvage old gears anymore. You get materials from killing mobs. They won't drop any loot, if you are 4 levels higher than the mobs you kill. So, you have to level your profession while leveling with your character because, if you fall behind, you have to buy materials from other players or via Auction House to be able to level Leatherworking.