I guess the first question would be why you want a custom? People have different motivations for going that route.
Personally I would think one of the better reasons is because there is a particular luthier that a person is drawn to, but still wants particular woods/features enough to have a ukulele specially made. Typically that person would do a lot of homework before commissioning the instrument.
Then there is a person who just wants something different and not the typical production ukulele. I tend to follow this path and will go a little off the grid for a ukulele as long as it is a quality instrument. Sometimes these instruments can be a little more affordable.
You may want to see if there are any local instrument guilds/associations that may be able to direct you to a luthier that will meet your needs. An example of this is SIMSCal.
Good luck with your quest.
John