I am assuming that you are concerned that you may be buying a pirated copy bubbles.
If so I would suggest that you actually buy from some one like Amazon.
http://www.amazon.com/Microsoft-Office- ... B0036Z0NW6although, of course, there is then the chance that you will be hit with customs duty on delivery.
As you can see it is not a cheap product to buy (385 USD, over 11,500 baht).
It is not available to buy directly from Microsoft unless you are a 'volume' (business) purchaser.
That & its' expense is, undoubtedly, why so many people download illegal torrent versions for free, & there are plenty of versions available over the internet, not that I am suggesting that you do that as it is certainly illegal.
I would, however, strongly suggest that you look at & consider Libre Office.
http://www.libreoffice.org/It is, legally, absolutely free & open source software which means that there is also huge numbers of free additional systems & programs also available for it.
Except for a few specific details it is almost identical to Microsoft Office.
I have noticed that it does lack some of the subtle features available with Microsoft Access for example & suspect that the same is probably true in the other products, but for general use it is pretty indistinguishable.
Libre Office grew out of Open Office which is owned by Sun Microsystems. When Sun was bought by Oracle many of those working on Open Office were concerned that Open Source software & Oracle would probably not be a good fit, longer term. They split away from the Sun project & rewrote the software to become LIbre.
Open Office is still available for free from Sun.
http://www.openoffice.org/Although all the big Linux distributions that used to include it have now moved towards Libre.