The most compact option, but maybe not the easiest is to modify a PC-keyboard. I have modified an old Cherry AT keyboard. This keyboard has a 40-pin DIP 8051 inside. Modern keyboards might not include an 8051 or use a different case. This is what I did:

1) Draw the schematic diagram of the keyboard.
2) Replace the 8051 by a socket.
3) Write my own code for scanning and communications, including tables to relate scan codes with ascii codes. Plug the new 8051.
4) Replace the 2k2 pull-up resistors by 10k in the old clock and data lines. One line is now used to transmit, and the other to receive.

In this way, the keyboard can be connected directly (TTL levels) to an 8051 (not at very long distances). In my application the keyboard is powered by the external 8051 power supply. Using an internal CMOS 8051, the keyboard needs about 30 mA (about 80 mA with all LEDs on).

If you need an RS232 interface, you can add a MAX232, however this will increase de power supply current.

Regards,

Alfredo.