EVA foam's an awesome material for lightweight plate armor. Harbor Freight carries EVA floor mats, yoga mats are available in most store's sporting goods section, or you can just grab some of the thicker sheets of Craft Foam at your local Michaels/Hobby Shop.
I recommend contact cement for putting the pieces together, but it is possible to get nice seams using a hot glue gun.
If you can't find a template online, you can make one from your paper mache pauldrons. You'd essentially wrap the old pauldrons in aluminum foil, then wrap that with a layer of duct tape to keep its shape. That should give you a foil/tape form in the shape of your pauldrons. You then make as many incisions necessary for this form to lay flat. Boom - you made a foil/tape template you can trace onto your foam sheets.
Cut the foam along the template and glue all the seams together. If you're not using contact cement, what you'll want to do is run a piece of tape along the outside of your pauldron seam, then run hot-glue into the seam from the inside. Once the glue's cooled, remove the tape, and you should have a smooth surface.
The trick to making foam look like metal's all going to be in the paint work. If you managed a decent paint job on your paper mache pauldrons, you shouldn't have any issue with foam ones.