Check eBay, Reverb, or HiFiShark. Search for "Denon D-M707R parts" as well. Join the Denon Vintage Audio group on Reddit for specific refoaming guides.
Most Western audiophiles have forgotten the MD format, dismissing it as a relic of the pre-MP3 era. But for those in the know, units like the represent a peak of engineering that deserves a second look. denon sc-e727r