Waterbeds are produced in different dimensions than regular bed styles. The common king mattress is called a Eastern king, which measures 76 inches by 80 inches; however, waterbeds are considered California king and measure 72 inches by 84 inches. So, invariably you can't by standard mattress sheets to fit your waterbed because they would be too short.
The waterbed is characterized by its ability to shift when weight is set in one area. The shift in water to a different corner of the bed can cause regular sheets to be pulled up from a corner, leaving your mattress exposed. Specialized waterbed fitted sheets have extra-deep pockets that ensure your sheet corners will stay in place. Certain waterbed sheets also have a pole fitting device that attaches to your frame or top sheets that can connect to your fitted sheets.
Waterbed sheets are not hard to find. Most commercial stores that sell linens all carry specialized sheets designed just for waterbeds. These sheets all can be ordered from the Internet or picked up in-store. Waterbed mattress stores will also have a large selection of waterbed sheet sets and specialized bed frames.