My mom needs to get a cheap version of Microsoft Office Pro and all of the websites that carry old versions of it are pretty sketchy. Is there any website that it can be bought from that I can trust?
Does she absolutely require MS Office? You might want to take a look at LibreOffice (http://www.libreoffice.org/). It's free and will do most, if not everything Office can do, excepting run any embedded macros.
Are you sure she needs Pro? What apps from Pro version does she need?
Apart from that - eBay and similar sites is your choice (especially if you will need media, too, not just a key).
Does she absolutely require MS Office? You might want to take a look at LibreOffice (http://www.libreoffice.org/). It's free and will do most, if not everything Office can do, excepting run any embedded macros.