Went to an AMC theater last night and they had some hard plastic Batman Returns cups. These are not a neat as the 7-11 cups as they only had one image on them and it wasn't a "3-D" effect. However, they were a little bigger and image they would be good for popcorn in a HT.
As I was walking out I noticed most people just leaving there cups, so if you are not disgusted by it you could collect a bunch. However, I just went out to where they sell the drinks and asked for a few clean ones "for my kids", the guy handed me 5 which was very nice of him. Of course YMMV!
Again, not worth a special trip or anything, but if you go to an AMC, and you were getting a drink anyway, get the large and see if you want the cup! BTW the large is huge, good for splitting!