VSTO, or Visual Studio Tools For Office is not a product per-se which you can purchase. Rather, it's a freeware add-on or Visual Studio containing callable classes and method "wrappers" specifically for the purpose of interacting with MS Office applications.
Where it is:
You can download it free here
http://www.microsoft.com/downloads/d...displaylang=en and don't need to buy a licence, though you will need a copy of Visual Studio to use it (ok just checked the requirements from that page & you'll need VS.Net 2005 or above).
What it is:
If you're asking what it is, then think of all officve versions, 2007 (and the next one too) included as containing a COM based / VB6-style programming library and interface - i.e. you hit Alt+F11 in any of the office programs in any version and you're presented with a VBA editor which is remarkably like VB6 designed for interaction with the office apps - not .Net like at all.
VSTO provides you with libraries which you can use in VS.Net. Once added, you can type Dim xlObj as Excel.Application and all the methods and intellisense you're used to in office are there. Behind the scenes it takes your .Net written code and converts it to old style COM code to run against the office applications.
2003 / 2007 specific versions:
Finally, if you're asking where the 2007 specific version is - take a look at that link, the 2003 and 2007 office versions can both be interacted with, by using this same, single version of VSTO.
Hope this helps.