Are there sources for high resolution scans or photographs of famous
historical paintings?  There are a few companies offering reproductive
prints of famous historical paintings.  Where do they get their
digital scans?

They get there digital scans from, or are given permission to take
scans/pictures by the owners of the paintings. This is an example of a
copyright notice from the National Gallery in London.

"General Copyright:

The National Gallery holds the copyright for all the photographs of
paintings in the permanent collection displayed on this website, and
always needs to be contacted for permission when paintings are to be
reproduced. However, for some more recent works in the collection the
work itself will also be in copyright.

Artistic works are protected in the EU from creation for a period of
the lifetime of the artist plus at least 70 years. If you wish to
reproduce a work in copyright you must by law clear permission with
the rights holders (usually the artist or their estate) for the
artistic work, and the Gallery for the photograph."
http://www.nationalgallery.org.uk/home/copyright.htm

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