I have never had a problem finding fitted sheets, you may just be looking in the wrong stores, or the wrong area within the stores.
If you go to a specialty store, like Linen & Things, they are very easy to find.
That said, the flat sheet could always be used in place of a fitted sheet, though it won't stay on as easily. They have elestic straps that could be used to keep them on though, so you purchase a full sheet set, get 2 sheets to use on the bed, plus your pillow cases, for probably the same prices as one fitted sheet and the pillowcases.