Replacement sink plug weird size - can't find a 22mm plug anywhere

Hello,

This one goes out to the caravan owners, does anyone know where I can buy a replacement sink plug? The one that came with our van doesnt fit and the smallest I can buy is 25mm which doesnt fit. I measured the hole and its 22mm across. The online stores I have checked dont sell any smaller than 25mm. It seems weird.

thanks

Hayley