Castelvetrano olives come from Valle del Belice, a region of Southwestern Sicily. These are medium sized green olives with a rich buttery taste. Make sure to purchase these olives from a reliable supplier to make sure they don't dye them. If the olives appear "nuclear green" avoid them! Castelvetrano olives are typically used for making olive oil but they've become popular recently as a table olive. You can buy them with or without pits and they are typically packed in water. A reliable supplier is The DeLallo Company
Look for these olives in a good deli or a store that specializes in Italian foods. You may find them at Trader Joe's or Whole Foods Market too. You can always purchase them online at Amazon.com: Whole Castelvetrano Olives