The heavy duty JR servo arms are part number JRPA215.
They go for $2.50 a pair at your local hobby shop.
They cost about $8.99 a dozen.
The super duty white servo arms are available too...they feature a metal ring around the splines, and are standard with the high voltage servos 8711HV, 8717HV and 9100HVT/S. The set of two arms , cushions and screws sells for $8.99 They are pictured below.


I have found that the major problem with most of the metal servo arms, is that they are drilled and threaded for a ball link. The threaded holes are too large for a Sullivan steel clevis pin.
The JRPA 215 clevis holes are molded to fit a clevis, and are very strong. The weakest part of this servo arm is the splined hole.

4-40 is the appropriate size for model jets.