Ralph Lee Smith has his second edition of Appalachian Dulcimer Traditions out now which is a good resource on the history of the dulcimer and good information on some of the early builders. His website is http://www.ralphleesmith.com/Books___CDs.html.
Kevin Messenger, John Knopf, and Dan Cox, all members here make dulcimers in the pre revival style and know a good bit about some of the early builders. There are many builders of modern instruments all across America and some in Europe as well.