Did you ask if clerk could tell you where to find one, or if they could get it from another store? It might be that you ran into a clerk who just didn’t know how to get one ordered and didn’t want to be bothered to find out, or they really might not have a system that allows them to do that from the sales desk.
Unfortunately I’d vote for the former, but if you really want the thing it might be worth asking if they can get it for you from one of their other stores. Of course, if you go back you might also get another clerk who knows what he’s doing. Or cares.