Had that problem at my local Michael's, but there is another Michael's located about 25 minutes from that store. I asked if they could call to make sure there was stock in both sizes before I drove over and they were nice enough to call. They had three of the small and about four-five of the 1:18 size...I went and got what I needed.
I'm sure you've already checked, but just in case I'll ask...are there other Michael's locations that aren't too far from you? Also, if you can call other locations and ask (need stock number) and see if they will ship them to you from the store.
Not sure about the others, but I do like the Michael's cases. Of course, I don't think they're always on sale and, at regular price ($18 for 1:18), probably better off ordering from somewhere else.