Wow .Great deal .I picked up the Sony MZ-R900 new back in 2001.It works great and sounds great.I used it for live recordings and off air radio recordings until I got my digital SD card recorder in 2007.I still used the MZ-R900 as a backup to the Roland Edirol R09HR digital recorder until I semi retired it 3 years ago when I replaced it with a Sony PCM-M10 recorder.As that that time it was a pain in the rear looking for blanks and also the internal flat gumstick battery was failing to charge.I got my last batch of blanks from B&H Photo in NYC.I dont know if they still carry them now.
http://www.minidisc.org/part_Sony_MZ-R900.html
Heres info and service manual on your model.
http://www.minidisc.org/part_Sony_MZ-N707.html