updated by @ken-hulme: 08/11/23 04:35:45PM
Welcome to the world of dulcimers! We can probably help you, but we're going to need to see what's what.
You may not, in fact, be "missing" any frets. Older dulcimers -- what we call "pre-Revival" and dulcimers from the early Revival period of the 1960s, '70s and even some today, have what is known as a 'true' diatonic fret layout. More modern dulcimers have two and sometimes more frets not found on a diatonic fretboard -- but that does not mean the instrument is "wrong" or "broken".
Frank Bond was a "fashionable" and popular dulcimer builder in London in the late 60s and 70s. He built a number of different styles/shapes/sizes of instruments with fairly long VSLs (distance between nut and bridge). He also built diatonic and "diatonic-plus" fret layouts. His instruments are highly regarded by dulcimer collectors, especially on this side of The Pond, as they are few and far between. IIRC, there are a couple folk who are members here who own one or more Bond dulcimers.
We're going to need to see clear and relatively close up pictures of the instrument, particularly the tuning head area, tail area, and the fretboard itself, so we can determine whether there are indeed any frets missing.
New strings are simple, cheap, and easy to come by at a local acoustic music shop or online, and given the distance between the nut and bridge we can tell you what strings to buy and how to go about changing them.
Looking forward to seeing your Bond, Frank Bond (not James)!