Hi,
You will need to look for second hand copies, there are no new ones for sale. Early Windows systems will not load on most current machines as they do not understand the CPU's or RAM.
Either DOS or early windows will only load on drives up to 2 GB, and their fdisk program will not correctly address modern drives as the layout is totally different to the drives at the time. Neither has any support for tcp/ip, so internet connections are not possible.
I would suggest you to use your favorite search engine and search, you might get links to download the DOS.
**Note:**Using Third Party Software, including hardware drivers can cause serious problems that may prevent your computer from booting properly. Microsoft cannot guarantee that any problems resulting from the use of Third Party Software can be solved. Using Third Party Software is at your own risk.
Hope this helps.