I have a M50 and a M520 but I am not selling at this point. I would check E bay, US Audio Mart and Craig list as these things could pop up for pennies on the dollar but then there may be a lot to clean up on them depending on how they were kept. I had one studio with a M520 that never covered it when not in use and the dust that goes into them can cause much trouble translating into a lot of hours of fixing and many cans of Deoxit. If properly taken care of they can hold up.
I have had my M50 (the same as a M512 except for two pots) since I worked at Teac in the 80's.