This is the exact wire I use. It typically requires a slight enlargement of the hole on the servo horn, but fits the control horns nicely.
High Carbon Steel Wire, #2B (Smooth) Finish, Full Hard Temper, ASTM A228, 0.047" Diameter, 42' Length, Precision Tolerance
https://www.amazon.com/dp/B0036R4SWA/?tag=lstir-20