Where can I find a plastic cover for my tub drain?

Ok, let me make this super clear and precise. I do not need to know where to get the drain cover (the metal piece that keeps the hair out). I do not need a new drain stopper (rubber or plastic that goes down in a hole). My tub doesnt have a hole to put a stopper down inside. It has a metal cover with small holes. My stepmom years ago had a flat plastic cover, that kept water out of the drain so you could take a bath, because the drain stopper inside did not work. I want one of those, but for the love of God I can't find it anywhere online. HELP me please. It looks like a plastic pancake with a bullseye on it. Again it keeps the water in your sink or tub. You put it over your drain.