If you purchase a RiffTrax that does not have a .riff file or a PAL version available, they WILL be made available to you once they are added. So if you ever get one by accident, you will be able to get it as soon as you would otherwise.
Of course, there will always be the rare exception that does not allow us to create a .riff or PAL file, but they do not happen very often. It is probably best, if you require one of those two formats, to wait until it shows up on the product page.