Juried Engineering Lite-On 4N25 Optocoupler, Phototransistor Output, with Base Connection Breadboard-Friendly IC DIP-6 (Pack of 20): Amazon.com: Industrial & Scientific
Amazon.com: AVLIS-CO Pack of 2 4N25 Optocoupler DC-in 1-CH Transistor with Base DC-Out 6-Pin PDIP : Industrial & Scientific