How do I buy Microsoft Word only?

Go to the microsoft store for your country.  The MS store does have standalone copies of each of the applications it is just difficult to find them.

Here is a link to the page in the US MS Store:

https://www.microsoftstore.com/store/msusa/en_US/list/categoryID.69404600

If you are not in the States, you can try changing the country in the bottom left corner of the page. But it probably will not stay on a similar page.

Note: If you are buying if for personal non-business use, then scroll down, near the bottom of the page there is a cheaper "non-commercial" license for Word.

I agree with Doug. Think carefully before buying. If there is any chance you will need Excel or Powerpoint you should buy a copy of Office. The price is only slightly more if you buy them now in an Office bundle.

You can get OneNote free if you want it..

Google for Buy Microsoft Word.

In Australia, that brings up a site where you can buy just Word for AUD146, but at other sites, you can buy Office Home and Student for AUD124, which will also give you Excel