I'm not sure either of these options fits your heli:
These are not cheap, but they do work rather well and are very hard to break. http://www.helipal.com/3-x-airy-hard...-ii-ax-09.html
I had a Walkera 38# Tandem coax that I would often get blades strikes that broke 5 or 6 stock blades. With the AH II blades I only broke 1, nicked a few though.
If you aren't inclined to spend that much, I think stock Walkera 400 or ESky Big Outdoor Lama Blades would work: