If they don't sell "official Watchmen smiley pins" in stores now (probably comic stores or novelty stores like Hot Topic), they will soon.
Failing that, you could probably get a hold of a normal smiley-face button from somewhere, and add the blood splatter yourself using paint. Use red paint mixed with just a bit of blue (sounds weird I know) to get the blood color.
If you can't find a smiley button at all, you could make the face itself out of construction paper or something. Then if possible, get access to a button-pressing machine and make the button. I don't know where you would find one, but they exist. The crafts center at my school has some, for instance.