The Davis Square Community

City Sports in Porter definitely has them. You can take the T, if you don't want to walk in the heat. They have both the traditional waterproof kind, and lycra caps (which are great if you want to swim someplace that requires a swim cap for long hair, but you hate the kind that pulls your hair when you take it off.) Obviously, a lycra cap will let the water in, because it's just made of swimsuit material.