New Windsurfer One Designs are available through Windgenuity for $1799, fully rigged.
Second-hand boards are sometimes found for sale on Ebay but they are often sought after. If you're looking to get into a second-hand One Design you may be better off asking around your old windsurfing mates - lots of people have them under their houses. Old boards in good condition are still tough and very fast - mid '80s boards have won several recent national titles. New sails are available from Windgenuity for $475.
Junior sails are made by Barracouta Sails for around $300. They can fit on the standard boom and mast.