Is there a place where you can buy a plastic cap for a bottle?

Years ago Gillette sold soap and shampoo in plastic containers with hooks that you could use to attach them upside down to a shower caddy. I bought two bottles and have been refilling them. It was really convenient, but I have lost the caps for the bottles. Is there some place that sells plastic caps that you can snap open or closed to allow a liquid to pour through? Are there standard sizes?