There is a Yahoo group called publicdomaindulcimer, which has files of public domain songs to download. You will need to join the group to get access, but they have a good list of Christmas songs in DAd as well as hymns ond other stuff.
http://launch.groups.yahoo.com/group/pu ... ndulcimer/
There should be a sign up button near the upper left of the page that comes up with the link above.
I went back to look at the tabs, and some could be used by beginners by just playing the melody on the d string(s), while others require fingerpicking, or finding melody notes on the other strings.