Normally theres a listing of festivals here on ED but as Bruce and the guys are redoing things its not up right now. They post the ones theyre aware of but if the people doing one dont tell them it wont be in the list. A complete listing of every festival where dulcimers are involved would probally be so big would be a full time job to maintain.
Other than a couple like Winfield or Evart most people usally dont go to one much more than 400 or so miles one way so I would recommend checking here and the sites of some clubs close to where you live. The result is the festival lists tend to be regional except for here. There is a club list here also mormally and its the same situation as the festival list. If they dont tell Bruce the club dosent get on the list. While not actually a festival just about everywhere you will find jams if you ask around. Many are not dulcimer centered but are often dulcimer frendly. These are usally a few hours but some some of these could well approach being considered a festival of sorts too. Theres one near me that is "offically" a 2 day deal but it usally goes for the better part of a week. No vendors and no workshops scheduled but ask around and just about anybody will do a one on one or an informal workshop.