Tutorial: How to use Arduino Uno as HID | Arduino Keyboard Emulation - YouTube
How to Set Up a Keypad on an Arduino - Circuit Basics
16 Keys Matrix Keypad 4 X 4 Membrane Keyboard Module Array Switch for Arduino UNO R3 WIshioT : Amazon.in: Computers & Accessories
Vanki 4 Button Key Module Switch Keyboard for Arduino UNO MEGA2560 Breadboard 2pcs - Amazon.com
How to read input from USB keyboard on Arduino
3pcs TB371 4 Key MCU Keyboard Button Board Compatible UNO MEGA2560 Pro Mini Nano Due for Raspberry Pi Teensy++ Geekcreit for Arduino - products that work with official for Arduino boards