I'm working on these at the moment.
The masks ARE generic Halloween masks, but they're a thin hard plastic rather than latex, and they've been modified. They've enlarged the eye holes on the wolf and tiger (the sheep has been left as is), added nostril holes to the wolf and repainted them.
These are the exact masks that they modified (I've just recently purchased these):
The You're Next Facebook site has some good images that can be used as reference for the eyehole shapes and paint. I'm planning on spraying them a flat white first then adding the other paint details, weathering and blood splatters.
There were a whole load of replica masks given out at SDCC to movie viewers, the replica masks have also been mailed out to movie-related press and others were placed on statues all over New York. I've studied them and while they are very good, the eye holes have been altered into the wrong shape on all of them. I'm also not convinced that the paintjob is exactly the same as the ones in the movie. These are the SDCC promo masks:
You can see that the eyes are almond shaped (should be much rounder near the brow). The sheep mask has dark paint detailing around the edge of the face that is not on the movie mask. The tiger has very wide dark paint detailing below the nose. Again, this is not like this on the movie mask. And so on.
I'm just not sure whether I should put down a layer of fibreglass and tissue matting inside each mask before starting the painting in order to reinforce them (the plastic is very thin). It would make them more sturdy, but it may make them too heavy to be held on with the elastic.