In the movie, it is shown that the operation and suit-up of Vader begins shortly after Palpatine finds him. I was reading an article explaining Vader's armor and it shows that a lot of thought has been made into every single detail of the armor, like the the edges of the helmet being made to deflect blows to the shoulders, the breathing aparathus, the whatever in the feet, eyes and so on.
Furthermore the helmet and shoulders are made from durasteel (among other metals) and the hand is forged from sith amulets.
The question is simple: How the force did Palpatine have such a suit lying around ? It looks like something which will take a while to create even if all the materials are avaliable.