That should be a standard GM radio connector, so sources like Classic Industries, Year One, , and others probably have the connector. However, the year and trim level may make a difference, since early trucks usually only had an AM mono radio.
Also are you talking about the power connector or something for speakers? The early power connector is usually three wires, with ground, 12 volts switched, and instrument panel dimmer. I assume they later added a wire for 12 volt unswitched (for a clock the radio).
Bruce