You can get lots of MS-DOS boot disks here.

I doubt you really want the original MS-DOS -- DOS 1.0 didn't understand things like 1.44MB floppies. Win98 had a genuine MS-DOS underneath, which makes it probably the newest version of 16-bit MS-DOS except for the crippled version underneath Windows ME.

There's also FreeDOS, which is MS-DOS compatible. I've seen board manufacturers use it for BIOS upgrade disks and the like, so it must be pretty stable.