Haven't seen any in stores. Here's an online inquiry on the General Board where more people might have answers for an online source.
Maybe if there is a local restaurant that uses them you could ask where they get their source. Thomas Keller once had them on the menu, so maybe writing the French Laundry you can ask where they order them. Probably another restaurant to inquire is at Olivetto.
What are you planning to use them for? Or are you just curious about trying them?