Strings
General mountain dulcimer or music discussions
Skip has it right. Check out Get Tuned for a visual depiction of the tuning of the dulcimer relative to the piano.
A couple of things to remember:
1) The suggestions you've received so far for string gauge are reasonable, but without knowing the VSL (vibrating string length, or the distance between nut and bridge), no one can really know for sure what gauge strings would be appropriate.
2) Your mileage may vary. Play around a little to find your personal preferences. I have discovered that I like slightly heavier strings than most, at least for flatpicking. But I often tune down to C for fingerpicking because I like a little give in the strings, something I definitely don't want when flatpicking.
3) Steel is steel and strings are strings. Don't worry about brands. Just figure out what gauge you want for each string and buy single strings, avoiding sets which might not have exactly the right gauge for each string and also cost more per string. Once you know what gauges you want you can buy in bulk and save even more.
3a) The exception to the statement above is that wound strings come in a few different varieties. The most common are nickel wound and bronze wound. Take the time to discover your preferences. Personally, I like the bronze-wound strings because the tone is more mellow. The nickel-wound strings have a brighter sound, and that might be more appropriate for some dulcimers than others and for some pairs of ears than others. Also, if you find you get a lot of squeaking on the wound strings, you can get "squeakless" strings (which aren't actually squeakless, but the squeaking is reduced). I won't go into the options there, but just know they exist in several different varieties but some people think they produce a more muted tone.