You can get the official Microsoft files (like io.sys, msdos.sys, command.com, edit.com, fdisk.exe, ...) from these sources:

• boot floppy images (extract files with WinImage)
  • ms-dos7.hit.bg
  • bootdisk.com
  • allbootdisks.com
• CD ISOs (extract files with 7-zip)
  • ms-dos7.hit.bg
  • allbootdisks.com
  • bit torrent

If you have Windows 95 or Windows 98/SE install disks, you may be able to extract the DOS files from them.