The advice others have given to find a shop that has many ukes and just play them is spot on. But I have to ask - what are your reasons for wanting a custom?
I have two factory ukes (Kamakas) and one custom (DaSilva) and honestly - while they are all different, in my opinion they are all of equal high quality. My custom serves a specific purpose - I needed a 19-fret soprano with perfect intonation, and I got one - but had I not needed the extended fretboard, I could have happily gotten by with my two Kamakas. Both of those get played as much as the custom - I love them all equally.
This is not to say that all off the shelf high-end ukes are equally good - they aren't, which is why I went custom in the first place (high end factory 19-fret sopranos do exist, but I couldn't find one with the right sound and with good enough intonation). But if you shop around, you might just find the right one!
As for buying a used custom - I just don't get it unless you are a collector and really want a uke by so-and-so in your collection. I mean, it kind of defeats the purpose - a used custom was made to someone else's specifications, not to yours.
I think you're going to have to make the long journey to McCabe's one of these days, BIGDB