Raspberry Pi Documentation - Raspberry Pi hardware
40 Pin GPIO Extension Cable for any 2x20 Pin Raspberry Pi [150mm / 6 long] : ID 4823 : $2.95 : Adafruit Industries, Unique & fun DIY electronics and kits
Amazon.com: 2X 40 PIN GPIO Header Adapter Color-Coded Header Expansion Board for Raspberry Pi 400,Easy Expansion : Electronics
$7.95 - Black Raspberry Pi 40 Pin GPIO Extension Board - Tinkersphere
Amazon.com: Micro Connectors Raspberry Pi 40-pin GPIO 1 to 2 Expansion Board (RAS-GP02),Green : Electronics
40 pin GPIO Reference Board For Raspberry Pi B+/Respberry Pi 2 - CodeScript
RasPiO Portsplus 3 fits Pi3B plus and all 40 pin Raspberry Pi – RasPiO Store
Simple Guide to the Raspberry Pi GPIO Header - Raspberry Pi Spy
Raspberry Pi and General-Purpose Input/Output
40-pin GPIO Header for Raspberry Pi (color-coded) | Elektor
$1.99 - 40 Pin Leaf Pinout for Raspberry Pi GPIO - Tinkersphere
PiBorg | 40-Pin Female Header
Raspberry Pi B+ GPIO Header Details And Pinout - Raspberry Pi Spy