Where can I get that plastic thing that covers a tub’s overflow drain so I can fill the tub higher?

I saw this thing in a catalog years ago and have never seen it since. What is it called? Where can I get it? It was a translucent plastic "cup" which you fit over the bathtub’s overflow drain (the round thing under the spigot, where the drain-stopper switch is also usually located) which has openings at the top. This way, when the water level rises, it gets higher than is usually allowed, and then when it gets to the TOP it pours in and then through the little overflow drain. Can anyone help me? I need one! My life will be empty until I find it again.

I got mine at a hardware store.a big one like Home Depot or Lowe’s.