You can purchase just Word, but you will pay way more for Word alone than if you purchase the entire Office suite. It sounds ridiculous, but it's true. It's just the way Microsoft packages and sells their product.
Currently there are two versions of Office software:
1) Office 365: this is the cloud version. In this case, you are actually renting the software. You can pay a year up front, or pay it monthly, but either way you are bound to a year contract. But there is no software to have to install as everything is maintained on Microsoft's servers (i.e., the cloud). I personally don't care for this version because I want to own my own software. But that's just me.
2) Office 2013: this is the current desktop version. That means, you buy the software and you own it outright. You install it on your computer just like any other version.
If you prefer one of the older versions (Office 2007 or Office 2010), while they have been discontinued in as far as no new copies are being manufactured by Microsoft, you can still purchase either version from various online Office supply stores who still have some copies in their warehouse. You can do a search (yahoo, google, etc) to see who is cheaper. But I'd start at amazon.com because their prices are hard to beat
Two other options:
1) If you don't to buy anything, you can use Microsoft's free version of Office 2013. It's still the cloud version, but it's free. Why is it free? Well it's a very basic version of the software, but it has all of the features you would need as an average user. To use it, you just need to set up yourself a microsoft account (and anybody can create an account). That is here:
https://login.live.com/login.srf?wa=wsignin1.0&rpsnv=11&ct=1369353167&rver=6.2.6289.0&wp=MBI_SSL_SHARED&wreply=https:%2F%2Fskydrive.live.com%2F&lc=1033&id=250206&cbcxt=sky&mkt=en-US&cbcxt=sky
2) If you still prefer the free route, and you want to have some software installed on your computer, you can get one of the generic office programs. They are all pretty much the same. About the biggest difference is in how often the vendor updates his software. So IMV the best of the lot is Libre office. It has an entire suite of programs that will edit or create any Word, Excel, Powerpoint or Access file. That is here:
http://www.libreoffice.org