You can pick up leftover cardboard from Costco or BJs. You'll need to find big, flat panels for the chest piece. Creases in the middle will make things more difficult and ruin the look. You'll also have to find some kind of softer, more malleable cardboard to use for the curved part of the legs and the headpiece. This is imperative, because rigid cardboard will not work for these pieces. You'll need a good amount of black spraypaint for most of the costume. Also, one can of silver spraypaint, and some acrylic paint to do the design on the chest. The rest is good, old-fashioned elbow grease and a little creativity.
I took my body measurements and also all the dimensions of the little LEGO figure I bought, then tried to scale it up. Some things had to be modified to fit a human body, but I tried my best to keep the proportions as close as possible to the actual figure. The hands were cardboard reinforced by papier mache, and the headpiece was a cardboard frame (using that softer cardboard I mentioned) with lots of papier mache. I then used black pantyhose to cover the eyeholes and the ventilator. Add a leftover cape from a previous year's costume, and you've got yourself a LEGO Darth Vader!