I play in a variety of tunings, but mostly DAD (myxolydian), as it seems more versatile to me. One must use caution in using various tunings with strings which are for a specific tuning. If you play mostly in one tuning, get those. If you vary alot, try to find keys which will cause the least overstretching of strings. When I retune my DAD to DGD (Reverse Ionian), I have great results.    For mixolydian, use these gauges or close to it: .023 wound bass, .014 plain middle, .010 plain melody - D-A-D tuning. There are some who are diehard DAA players, so for DAA (Ionian) use these gauges: .022 wound bass, .012 plain for middle & melody.
I have found that McSpadden Dulcimers have a nice variety of string set. Since I got my Dulcimers from them, and have met them, & liked the strings that came with my new Dulcimer, I just got into the habit of getting them from The McSpadden bros. I'm sure there are plenty of others. You can use individual banjo or guitar strings, just get the gauges you need.
    I now have 2 dulcimers, so I keep one in DAA, & the other I use for DAD & DGD.   There is usually a Mountain Dulcimer group around the country, so you might check one out. For me, it is lots more fun to play Music with others. Good Luck!