Some devices are ready to receive an Electret microphone, actually the
most common and cheap microphone. You can try to plut the mic directly
and see what happens.

If it doesn't work, you will need a 3V battery (at least), one resistor
(5k to 10k Ohms) and one capacitor (100nF).

      +3V  (2 small watch cells)
       |
       R 10k
       |
       o----||----> Signal
       |   10nF
    \.---.
     |   | Mic
    /'---'
       |
       o----------> Gnd
       |
      Gnd

You will want to install a power switch.

If you are handy enough, open the microcassete covers, and install the
resistor and capacitor directly inside the unit, using the unit Vcc
power.

Another solution is to buy a cheap computer mic that comes with the
battery compartment and power switch at the back of the mic.  Less than
$6 around.