What sources can I use for weather data?

Refer first to the NOR and SI's.  In general, they are expected to say that you may get data that is freely available on the web, but may not be processed or selected, or annotated for you by somebody on land.  This means no weather router once the prep signal is up.  And nothing that you have to pay to get the information.

This does not rule out a service that you pay to access generally-accessible web data.  Examples are the not-for-profit Sailmail Association, using your SatPhone, or some other paid data carriage provider.  What would be forbidden would be a service that provides you some "exclusive" content.