I used trash cans from walmart at about $5.00 a piece for mine. I bought three. To bend it, for the knees, I held both ends together with a huge wrench and just kept it in boiling water for a little bit, took it out and put it in cold water and its pretty good now.
For gauntlets I made them out of matte board from Hobby Lobby which cost about $3.00 and they look fabulous. I found some old hobby paint that my mom had and used that on them, not totally accurate, but spray paint isn't cheap. Note, this was for a ROTJ.
If you are doing a custom I don't see what's wrong with using sports pads or shin guards. If its a custom, you should try to have a different looking costume.
For the vest, I used an old over-sized tshirt and cut off the sleeves. I then used a thicker tshirt and made a second layer of the sleeve. I drew the lines where the seams were between the puffiness. Then stuffed the sleeves with old, clean sock material. It turned out neat.
For this project I would keep your eyes peeled for anything useful lying around the house, cheap finds at any store you go to, and the scratch-built threads here. Good luck!