Without knowing where your departure and arrival points are it's hard to offer detailed advice. I have no first hand experience of ocean passage
weather so take the following with a grain of what ever.
The best weather forecasts come from the internet
as you probably already know. Some seem quite reliable for 3 days but it starts getting more and more fuzzy as the forecast
period extends. It would be a good idea to familiarise yourself with the idiosyncrasies of any that you plan to use.
If you have a SSB receiver it can be connected to a computer to receive weather faxes and forecasts. This can be tricky so again it's a good idea to try different software
and to check that everything works. There are BOM marine weather forecasts forecasts from Australia on a regular schedule.
probably has something similar.
This site has some useful tips.
I've seen a few good comments on Passage Weather
However from reports it would seem that custom weather reports from an experienced personal meteorologist are the best way to go. Something like what is to be found here
. You'd probably need a satellite phone
I have a hazy recollection of their being some New Zealand custom weather forecasters but can't find a website.