Howie Mitchell wrote a neat little book on building hammered dulcimers. Don't know if it's still in print.

I built some from scratch, mostly using hard or curly maple for the sides and inner supports, and black walnut, cherry or curly maple for the pin blocks. For the soundboard and back, I used thin plywood with a decorative wood veneer.

Never buy a hammered dulcimer kit--they're all lousy. Never buy a dulcimer you haven't played--there's too much variation in sound. Pay particular attention to the bottom notes, as these are the ones you'll have the most trouble with, and play some runs quickly, so you can tell whether the sustain is too great.

Also, plan to spend as much time tuning the damn thing as playing it (though playing blissfully out of tune seems to be traditional practice).