This is a part to buy second hand or new. However Windsurfing masts are getting close to Moth Masts and if you can get a stiff one or add carbon to one then you cound save cost. These can also be sourced second hand.
" Mike Cooke Tushingham freeride 75% carbon (520 long, just a touch short to put a KA sail on, but they do a longer one) - any good windsurfing store, but I got mine from Nobleboards as they're local. They do a superlight one which is a bit more expensive, but I will be going for that on my next boat. "
The manufacturer of the Bladerider Mast is CST Composites and they can be contacted directly http://www.cstcomposites.com/Moth.htm.