I've gotten many of these. They are at the theaters near the desk where you buy your movie tickets. They appear to come in two different sizes (depending on film) - the 17 x 24 that you mention and 11 x 17. As you said they do appear to be about 50 of them that the theater gets for each film. Some films never get these mini posters. I don't know much more about them other than that.
~ Juli
Thanks very much for clarifying, that's what I thought. A few for Raiders popped up and I was considering buying one because they aren't folded and I thought I liked the format better than the daybill. But honestly I think I'll still opt for the daybill because A) It doesn't feel as much of a novelty and B) I like the text wording better.