Aloha Scott,
There are two, and only two, music stores I'd recommend in Hilo. I won't refer you to gift stores or furniture stores. Hilo Guitars and Ukuleles will have the best selection and is the best music store on the island. Lucky you, they are open on both Saturday and Sunday. Music Exchange, a few blocks away, is worth a visit as well.There's also Pacific Music but I don't know anything about them.
There are a few builders you may want to call to see if they have anything available. As mentioned, there is Bob and Robin Gleason in Kurtistown, Crist Pung in Hilo, Dennis Lake is in Kau, midway between Hilo and Kona. On the Kona side, right in Kailua/Kona town, you'll find Music Exchange again, and another shop who's name either slipped my mind or I never knew it in the first place! In Holualoa you'll find the Ukulele Gallery. Call him and ask what he's got available and to make sure he's there. A little south of Kona in Kainaliu there is Just Ukes, a pretty good store with a wide selection. Just a couple of miles south of there is another neat little shop, don't know the name again, owner's name is Brian though, it's on the left hand side as you're heading out of town.
I'm sure there are others I've missed. Oh yeah, there's me too. I should be around all week except for Saturday. I'm right near where the lava flows into the ocean so you'll probably be down this way to see that anyway. Give me a call (phone# on my website) and I can give you more information on these stores and builders. By then I might even remember their names.
Have a safe and fun trip.