Firstly I would have a look inside Rick's online shop above. Then if that doesnt work for you, you can obtain kevlar/carbon tiller extensions from CST (http://www.compositespars.com/)or Ctech (http://www.c-tech.co.nz/producttube.htm) or Windrush online. If you are really interested in carbon/kevlar you can also try the 49er or Ronstan supplier in your country.
Look also in hardware stores for aluminium tubing or look up the "yellowpages" If you choose the aluminium just dont straighten the aluminium on the water if it bends wait till your ashore.