Au contraire Cactus - there is not such thing as "guitar" strings. There are just musical instrument strings packaged for various instruments. And you can find Dulcimer string sets at any reasonably stocked acoustic music shop, or on-line at www.juststrings.com
GailR - the string which you need depend on what tuning you want to use and the VSL of the instrument -- the distance between the Nut and the Bridge. However, knowing you have a FolkRoots dulcimer, and on the standard Dulcimer string sets will work just fine for all the standard tunings. Cactus was correct to suggest that you not remove all the strings at once. Change one string at a time. Removing all the strings at once can cause mis-placement of the Bridge, which can impact the trueness of the scales.
Not having a 6+ fret means that you may want to tune to DAA and use that tuning for many of the songs which are tabbed in DAd do not use the 6+ fret. These songs are in fact are Ionian/major scale songs not Mixolydian Mode tunes. You'll play the same notes as people tuned in DAd, but not on the same frets.