I work with the National Park Service at Morristown National Historical
Park.  We have three different methods of distinguishing our volunteers
from the visitors.  The first is a hat, bright yellow that has our
distictive NPS arrowhead patch with Volunteer written on it.  People can
see it and if we, as employees, must point out a volunteer, then it is
easy to find in a crowd.  For those who are not hat wearers, then we do
provide a neutral grey vest with the same NPS patch.  The vest is kept at
the site, so the cost is minimal, but it is something that is very
distinctive.  Since we do deal with 1780, the park is also willing to loan
period clothes (civilian or military) to any volunteers who are interested
in doing it that way.  One section of the park we do living history, that
is how we dress as staff
and that is how we expect our volunteers to dress.  But the other section,
we provide guided tours and we as staff can wear our period dress, or our
Green and Grey NPS uniforms and the volunteers may wear period clothes or
they may use one of the alternative above.  These are the methods that we
use to help
distinguish our volunteers from the public.

Other sites that I have worked at have had polos (short sleeved shirts with
colars) that I think are very sharp looking, those who do not feel that
they look good in a T-shirt seem not to mind these, and they are very
distinctive for the public to see and for the staff to find.  If I were
looking for a uniform for volunteers, I think that I would go this route.
If they wear them to other places, what better advertising?  Naturally you
would want them to use better judgement when doing this though.  It is up
to you whether you want the volunteer to purchase it, you want to give it
to them, or you want to let them keep it at all.  The National Scouting
Museum in Murray, KY does not let the staff keep their shirts when they
leave.  They were the property of the museum and must be maintained by the
employee/volunteer and turn in when this individual no longer works there
in a clean condition.

Hopefully some of this will help and good luck in your search.

Michael Hosking
Park Ranger
Morristown National Historical Park