You can get sliding track cleats at any number of places. I bought two stainless steel ones (I can't remember from whom) and they work great. Excellent investment in flexibility. Check West Marine and Defender.
BTW, the cleats over at sailcare aren't going to be as good or as durable as the Schaefer cleats, since they're a mix of aluminum and stainless steel, where the Schaefers are all stainless steel. The Schaefers are also designed to work as chocks, which you can't do with the sailcare rail cleats.
The Schaefer cleat allows you to use the cleat as a chock or run the eye of a dock line through it and over the cleat. It is also easier to adjust, using a spring loaded pin, rather than a thumbwheel. Finally, it is a single piece of investment cast stainless, not assembled from an aluminum cleat and stainless base.