Word of mouth.

The point is to realize that almost every non-IT company of certain size (lets say 50-100 employees+) that started before SAP&similiar became popular in your country has large body of custom software and that the guy who built it is probably thinking about retiring.

Minus is of course that most custom software is pretty horrible from code quality standpoint, so you need to have a high tolerance for debuging and that you need to have a little knowledge of everything (specialisation is not possible).

You'd be surprised how many such jobs are there. Mostly not listed though. If my experience is any guide, one generally starts as a temp / consultant and you slowly expand the role from there. There's generally a point some years later where you're almost "too big to fail". Too many small odd jobs that are needed maybe once a year, but when they happen, they are pretty important.