No experience with getting the
builder to provide a tonnage certificate, but we UK registered a while ago and used
Google to find a local (New Zealand)
surveyor with the right affiliation. They did the
survey, sent their documents to the UK
head office, which then issued the certificate. Unfortunately for you, he was a sole operator and has retired since.
Note, you only need to do this if your boat is over a certain size (15m IIRC). With a smaller boat you self declare.