One of my IOM colleagues, Peter, pointed me to this website today to design and buy hull numbers:-

https://www.vinylletteringonline.co.uk/lettering-designer

Peter also found this website in Italy:-

https://www.aa-parts.com/en/attrezzatura-di-coperta/attrezzatura-scafo/accessori/numero-di-registro-scafo-numero-velico.html