www.offshoreelectrics.com has 4mm solid shafts....

http://www.offshoreelectrics.com/pro...cen-str-shafts

youd want to get a flex coupler to reduce vibrations....

since you will eventually upgrade to a larger motor (700?) a 700 ill have a 5mm shaft, so youll want to have a 5mm to 4mm coupler

http://www.offshoreelectrics.com/products.php?cat=68 (look down the page for the graupner flex couplers.....there is also a 1/8" to 4mm coupler that will work with 1/8" shafts, like the 540 has....

here is a line of threaded 4mm graupner props that will work w/ the 4mm prop shaft...

http://www.offshoreelectrics.com/products.php?cat=60