An aggravating truth about the Walkman Pro - one of which I still have, use and would recommend (if they still made 'em) - is that the mic input is a 1/8" stereo phono plug.
That means you need to use a 1/4" to 1/8" adapter for some mics you might otherwise choose, or some other cobble-up solution. Adapters are just one more thing waiting to go wrong, IMHO.

If it is still made (I got my latest one a couple of years ago), the Sony ECM-MS957 is a stereo condenser mic, battery-powered (single AA), that ships with a detachable cable sporting a 1/8" stereo phono plug. So far, I've had good luck with both sound quality and durability, though I do worry about the possible lifespan of the proprietary cable.

That was my solution, after some digging. So far, so good.

- George