Kauai is one of the best places to purchase these Niihau works of art. Niihau is known as the forbidden island because it was sold to a private party in 1864 and only native Hawaiians and the descendants of the original purchasers inhabit the island. Making these shell Lei's is an art that has been passed down on Niihau. The Lei's of Niihau are made from the highest quality shells that wash up on the shores of Niihau. Gathering and separating the shells is very time consuming and is one of the reasons that these works of art hold or increase in value. Make sure you buy from a reputable seller and you should get a certificate of authenticity that will tell you the stringers name. Below is a link with much more information about the history and where you can find reputable sellers on Kauai.
http://www.niihauheritage.org/index.html