They seem very hard to come by - most are 460 or 530 - but I'm looking for one to rig up an 8.5 raceboard sail. My 460 and extension aren't long enough combined. Please PM if you have one you're interested in selling. 
You can also buy "bits" which extend your existing 460 mast, you then use your existing mast extension into the "bit". Not exactly sure what they are called. Might be cheaper!
