Can I buy Microsofts Windows for MacBook at Apple Store (Icon Siam), Bangkok?

OK, on your Macbook, go to the AppStore and see if you can get it.

You can also visit microsofts website: www.microsoft.com

Depending on you needs, you could also give the free libre office for mac https://www.libreoffice.org/discover/libreoffice/ a try

( it works fine together with microsoft office files) or even use Pages, Numbers and Keynote (alternatives for Word, Excel and Powerpoint): these apps are already on your Mac

Are you sure you mean Microsoft Windows and not Microsoft Office 365 or Microsoft Office 2019?