As I recall, the floppies for the Atari were hardware
compatible with the PC floppies. I just grabbed an old
Atari 520ST disk (A-CALC PRIME by KUMA) and was
able to read the first sector. It had an unexpected format
type but that is easy to tolerate. The sectors are 512 bytes,
the same as PC. I also wrote a program a long time ago
to manipulate floppy sectors without reguard to file formatting.
So, If you have some sort of disk image, we can probably
write it to a floppy using a slight modification of my program.
But......before doing that you might want to ask around
on the various forums at:
http://www.atari.st/
I bet someone there would provide the disk you need.