If you are going for cardboard as the main material, you should check out the Master Chief costume made from it on YouTube
here and
here to get a few ideas. I've also seen one with female Commander Shepard from Mass Effect made out of cardboard.
Not sure what kind of paint you are using, but you might consider using spray paint. It will be easier and faster to paint that way. If you are looking for the metallic look, Wal-Mart actually has some pretty good metallic/silver/grey colors.
A good place to get your materials would be any craft store. For the weapons, go to the dollar store or any Chinese knockoff toy store and respray them with any leftover spray paint you may have from painting the armor (can't have real/metal weapons at conventions as they will either be confiscated or you will be banned from the convention). Make sure the gun has no moving parts (ie, if you use an airsoft gun, get a cheap one and tape/glue the magazine in it (or not have one at all) since they will even confiscate that).
As for the cardboard, try passing by your local grocery store or Wal-Mart/Target and go around the back to pick up some fruit boxes or any other material you may need as they just dump them out for recycling and could care less about it anyway (have done this from Safeway and Target many times). If not, check to see if there is a local cardboard recycling heap nearby. Make sure to disinfect, etc. as there will be germies on them if you do this :P
Hope that helped you a little bit. Haven't done anything armor-like but have put together other cosplay stuff... any more questions and I'll try to help ya :P
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