Try: http://www.ukplasticsfabrication.co.uk/acatalog/Acrylic_Domes.html
This was a recommendation of one of the JC entrants. But they also have a section on boat windows. It might be best to give them a ring. According to someone who has one on order they made the dome for Robin Knox-Johnson
They also turn up quite regularly at boat jumbles. New, they're around £130-£150, but I bought mine for about £40 at Beaulieu Boat Jumble. It needed some TLC with plastic restorer, but was otherwise fine.
Bill Churchouse (JC 2006 - see image in the reply to the Welcome to the forum posting) bought his in the St Malo area, where they are made. I'll ask him if he can recommend a supplier.