I went to a local metal distributor in my area. Look in the Yellow Pages under Aluminum or Metal or Steel to find a distributor in your area.
For my collar armor, I bought a scrap piece of 1/8" (.125") thick aluminum for about $12. For my backplate I wanted it a little thinner, so I got the next size smaller .090" thick aluminum. They had to cut a 2'x2' pice from a full sheet, so the price was a little more. It came to about $27.
Gator