The parts you need are not Nacra specific. The spreader boots are available at West Marine (SPRBOOTSS).
Also available at APS... with a better picture.

http://www.apsltd.com/c-7513-spreader-boot-by-seadog.aspx

BTW, I could never get a good seal where the spreader goes through the mast. I ended up getting a solid aluminum bar and had it welded into the mast. That gave me a good seal.