I picked up a lot of MO disks from Japan recently. 77 mixed 230/640/1.3GB disks, almost all were 640MB. Over half were new still.
They came with a neat 12 disk binder case of which I’ve never seen before. I paid under $2/disk including shipping.
Supply from Japan is readily available.
I recommend an MO drive on a bridge machine and a SCSI MO drive on a vintage machine. ATAPI versions also work on those machines with IDE.
Speaking of which, I should try modding a 630/640 to replace the floppy with an IDE Fujitsu 1.3GB. I wonder if the bus in these can support master and slave devices.
For the ATAPI version of the Fujitsu drive, I’ve found that the same MO Disk Extension linked elsewhere in this thread works as well.