I have always put wood blocks between my clamps and workpieces to prevent marking the wood. I see that newer clamps have rubber protectors for this purpose that make assembly much easier. Can I buy these protectors for my old clamps?

Not to my knowledge, but you are right that it would be nice to have these rubber caps in all your clamps. I have found two ways to accomplish this. Find a hard rubber or soft plastic material. You want something that does not leave color marks, so steer clear of black rubber. Polyurethane adhesive is able to glue rubber and plastic to metal. If your caps are soft enough to spread when clamped, use a softer polyurethane caulking as the adhesive. The longest-lasting caps will be ones that are grooved to fit over the metal face, giving mechanical resistance to slip so the glue has less to do. The second technique is to simply put a blob of polyurethane caulking over the face of the clamp and down around the edges. Let it set and then cut the face flat and about 3 mm (1/8 in.) thick.