These sources may be "authorised" but is there anything in the FARs
which makes the sourcing of weather data from other providers illegal?

No, you can check weather anywhere. If you do not check weather from an
authorized source (in addition to whatever else you do) then you would
be deemed to not have "checked all available information".

Jose
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Nothing takes longer than a shortcut.
for Email, make the obvious change in the address.