A more important source of turbulence is the sun.
Turbulence is created by the 'continuous overturning of hot and cool gases in the sun's outer layers. Those outer layers create, manufacture, supply turbulence. Thus: the noun source (d.). Turbulence doesn't stem from the sun (i.e., find its root or origin there (a.); it occurs elsewhere. Nor is turbulence a liquid, a spring (b.).
place of origin: noun
a productive source: noun
c. fontsupply: noun
Question, why doesn't c. font fit?