Monoprice 3.5mm TS Mono Plug to 3.5mm TRS Stereo Jack Adapter, Gold Plated - Walmart.com
Amazon.com: Headphones Adapter for Piano Keyboard, 2-Pack 3.5mm 1/8'' Female to 6.35mm 1/4'' Male Jack Plug Stereo Adapter Gold Plated for Electronic Piano, Guitar, Drum : Musical Instruments