Unusual instrument.....makes me think of Amish ladies having a sing-song on the porch anywhere in the Midwest...
Hmmmm.....anyway, I can picture the instrument but I have no idea of any dimensions.
There is probably some obsure shop in Chicago that are suppliers for Chinese made dulcimer pickups...of course the Amish won't buy them because they don't run on parafin!!
Worth having a look online though..
Otherwise go to the Rockshop or Beggs and try a regular acoustic guitar pickup for size and work from there.
There are a few good brands that will give you a good sound.
Perhaps find something that sounds good and then look for the right size on line....always cheaper!!
I'm sure there will be something.....
Otherwise, enjoy jamming in the Midwest!!