You have several options to get a ukulele fretboard:
1. Buy a ukulele or mandolin fretboard
There are some companies who sell complete fretboards with
inlays, pre-slotted for the desired scale length. These fretboards are not easy to get, and they are quite expensive. You can also ask for slotted mandolin fretboards. They have the same scale like soprano ukuleles.
2. Buy a guitar fretboard
Pre-slotted guitar fretboards are easier to get. Most guitar
fretboards are made for a 25.5” scale with 22 frets. You can just cut off the first 10 frets. So your nut is then in the 10th fret of the former guitar fretboard. This brings the scale down to 14.31”, and you still have 12 frets left, which is acceptable for a standard soprano ukulele. Also look to the chapter about Fret theory.
3. Make your own fretboard
You can buy a “Slotting jig”, a “Slotting box”, or make the slots freehand with the Fret theory. You should tape the area to the left and the right of the slot which you want to cut to avoid scratches. And also your saw-blade should have a mark for the slot depth.