Haven't been to either but have often wondered about Gilmore Music and World of Strings. If you do go, please report back and let us know what you think!
From LBC it's 30 miles to McCabe's but just 13 to Island Bazaar in Huntington Beach.
Feasible by bike if you are car-less and in good health. http://www.ukuleleparadise.com/about_contact.html
They carry things McCabe's doesn't: Worth strings for example. And Kanile'as.
I make a point of stopping in when I'm down there.
Good BBQ across the street too.
I would imagine there is a good general music store in Long Beach, let us know what you find..
Janeray's suggestions look great, and they are only a couple of hundred yards from each other.