I would like to add my experience with Issac store. He also checks UU time to time, so perhaps he will say Hi to everyone sometimes haha.
He is one of the most patient guys I have met (looking back on my 30+ emails with him back and forth). He responds fast and in a very detailed way. As many has said, he is fluent in both English and Japanese, and often time you will Japanese post on his FB. He has custom wood that could be used to custom order some K brand if you are looking for something special and do not mind spending couple hundred more. The store is kind of small and hidden, but Issac can definitely find you a perfect uke if you can wait for some time.
Beside the famous HMS and Ukulele Lab, I also want to mention two places.
One is Ukulele Store at Waikiki. The owner Eddie is a very nice guy. Even though my budget was only around $3000 but I played more than 10 ukulele that is in the $5000 - $10000 range, and he kept encouraging me to try more to find the sound that I am looking for. It is true that the price tag is higher in this store, but based on Eddie, he personally visits and picks out the ukulele himself every time. For example, as most people know, the same batch of 5 red label koaloha, 5 can all sound slightly to quite different. And Eddie will pick the best sounding one for him. Indeed, the instrument sounds spectacular. If the budget isn't an issue, I recommend you to visit his store.
Another place worth to mention is Ukulele Friend. The owner Shawn is very knowledgeable with custom and vintage. His store does not carry normal production models, but only high-end custom, both new and second hand. The price range is around $2000- $15000 with the selection of Moore Bettah, Devine, Lymana, Kinnard, Steve Grimes, Pepe Romero, ...etc, you name it. Shawn usually gets a few Moore Bettah throughout the year, so if you are looking for one, make sure to contact shawn. One thing to remember is that it is by appointment only, so remember to call him before you visit.