DD disks can still be purchased in bulk. As a user of MIDI equipment, I still need them and bought a box of 200 last month (here in the UK). I will look out and post the name of the suppliers later. Otherwise check with suppliers of MIDI backing tracks and ask where they get their supplies - that's what I did.

I checked with Roland and they have no plans for hardware upgrades to let their (now obsolete) equipment read HD disks.

It is correct that reformating an HD disk as DD will not usually work, because the narrower head only formats half the track width, the other half is not deleted and will may then conflict. Formatting an UNFORMATTED disk as DD should work, but you can't get those either.