What size washer to purchase for the pop up bath waste?

Hello I purchased a bath waste months ago and I only replaced it yesterday. The plug doesn’t close it keeps popping back up. After looking at it carefully I noticed it is missing the rubber pop up washer .. it was probably misplaced by the kids so now am unsure what size rubber washer I should buy for a pop up bath waste can you advise inside diameter is 42mm and the outside diameter is 69mm please advise what size should I purchase as I cannot find one with a 42mm diameter. Hope to hear from you soon thank you