I use the frames form old campaign signs. You can get cardboard (corrugated paper) that can replace the original stuff. I help clean up a local grave yard and get the stands that some people use for flowers also. They are kind of like a art easel made form wire. They get tossed out all the time so I thought recycling them wouldn't hurt any. I get tons of old boxes form a local Dollar store. They break down and separate it from their normal refuse and had no problem when I asked about getting some now and then.
For rifle targets I am a huge fan of paper plates, saltine crackers, and mints glued to cardboard, and 23 gauge hulls on sticks.