copper washer 2004 Cio

I have a slight leak from the drain plug after an oil change and after more tightening its still there only less. It obviously needs a new washer. I understand Renault do a kind of washer with a rubber "insert" but I imagine a good old copper washer as used for generations will work. My usual car parts place don't stock them. Does any one know the size? I notice on Ebay you can get packs of 10 for very little I just need a size. Can anyone help please.