Elderly Instruments in East Lansing Michigan is good. I bought my first vintage Martin from them, albeit over the phone (since I'm a couple of thousand miles from them). The person I worked with spent about a half an hour with me. He played three different Martins into the phone for me and gave me his thoughts on each. I was very pleased when I received the one he had recommended.
I've never worked with them, but Gryphon Strings in the Bay Area has some nice vintage ukes, and I've heard good things about them.
The only shop I've actually visited in person which carries vintage ukuleles is The Denver Folklore Center. They usually have a couple of old ukes, and are really nice folks. They have a FANTASTIC repair guy.
I don't know where you're located, but I would highly recommend Elderly even if you can't physically go there. They have a good website with all sorts if treasures!