Calrad 35-558S, 3.5mm stereo female jack to dual male RCA plugs. 6"

Calrad 35-558S -- adapter cable, 3.5mm stereo jack (F) to dual male RCA plugs, 6 inches long 

also available: 
GC 30-1565, 3.5mm stereo male plug to 2 RCA female jacks, 8"
GC 30-1812, 3.5mm stereo male plug to L+R RCA male plugs, 6' 

keywords: 1/8" phono