Welcome Joe.

I've been using a Sony model ECM-S5 for a number of years but it seems to have been recently discontinued.

There should be any number of suitable microphones. Look for a stereo condenser type, preferably with built-in battery. A friend of mine uses the Sound Pro types. Not sure which model. But have a look at Miniature Cardoid (Unidirectional) and Miniature Binaural (Unidirectional) types.

What kind of sound are you trying to record?

Basically Condenser types give great sensitivity and I leave my recording levels set mid-way. That way you don't have to boost the gain within the computer or in the amplifier. Boosting will always introduce noise. Try to avoid that.