How to purchase and install Excel 2007 standalone?

Hi,

We're considering a rollout of Excel 2007 to 10,000+ users who are currently
running Office 2003. a) Is a standalone version of Excel 2007 available and
if so, where? b) If yes, are there any issues we should be aware of running
a 2007 version of one product (Excel) in conjunction with 2003 version of MS
Word?

Or is it the case that we have to upgrade everything (Office 2003 suite) to
Office 2007 at once?

Thanks!
James