Several different sizes on this page:
http://www3.towerhobbies.com/cgi-bin...SM&C=MGA&V=FUT
I've used the metal Futaba arms and they are very stiff.
I can also recommend the MLP arms sold by Airwild Hobbies:
http://www.airwildpilotshop.com/More...=product&id=90
They mount "upside down" to get the arm in line with the servo output. Work with 4-40 hardware