As the recently resigned executive director of the Museum Trustee Association
(MTA), a national nonprofit representing over 7,200 museum trustees in
institutions throughout the US, I strongly recommend that nonprofit boards
have adequate Directors and Officers (D&O) Liability Insurance in place.
There are a number of good policy writers out there.  The general insurance
policy your museum has in place may have such a rider available, but as you've
found out, there are countless others out there.  You may wish to contact the
MTA and inquire about D&O options.  They can put you in touch with a national
firm they have worked with that pools small groups like yours so that they
have access to lower rates.  The MTA's national offices are in Washington, DC.
The number is (202) 857-1180.  Good luck in what ever option you pursue.

Phyllis Doak