Ballenger spars can build you one, they're the cheapest I found when looking for a mast
http://ballengerspars.com/ They quoted me $4000 for a 34' with all the bells and whistles. otherwise check with local boat yards and riggers to see what's out there. I've also seen masts, especially 20' to 30' on craigslist, ebay, and the "gear for sale" ads on this site. You need a comparable mast to the one that belongs on your boat, extrusion size/thickness, fractional or masthead, swept spreaders or straight. There's also a yard down in Florida that seems to specialize in used spars and spar material, sorry but I cant remember the name, can be found on ebay. I eventually found a suitable mast on ebay, payed $300 dollars for it and then spent twice as much to have it shipped, still cheap at that price.
Good Luck
Rich