Here are a few links for you:
http://www.gryphonstrings.com/
http://ukulelesource.com/
http://www.5thstringberkeley.com/
Gryphon Strings has some high end ukuleles (Kamaka, Compass Rose) and its lower end ukes are mostly Fleas and Flukes.
Ukulele Source probably has the widest selection. It sells Kamaka, KoAloha and Kanile'a as well as Kala, Ohana, Lanikai
and others.
In San Francisco,
The Aloha Warehouse in Japantown has a good ukulele selection, including Kamaka, Kanile'a, Kala and Ohana. There's also
Guitar Solo, which has mostly guitars and a small selection of ukuleles.
Another option in the East Bay is
Music Works in El Cerrito. It has a wide selection of Kala, Lanikai, as well as Fleas and Flukes, but not a lot of higher end ukuleles.
Good luck!