I would check out Na Hoku. I know they have a bunch of stores on Maui, like the shops at Wailea and in the Maui Hyatt Regency in Lahaina.
I've always thought the quality of the pearls were good and the prices fair. I like Na Hoku better than Maui Divers.
Enjoy Maui! If you can make it to Hana, you should go! Wainapapa state park is just beautiful!!