The bottom line
In the end, don't trust what salespeople tell you. Windows 7 PCs are out there, but you need to visit Dell.com, HP.com, and other PC resellers' websites to find them.
Learn to use the specification sorting tools that typically run down the left side of such sites. In those columns, you'll see filters that you can choose to narrow your search.
As you drill down to Windows systems, you'll see the Windows 7 filter. Click it, and the chances are good that you'll be able to avoid Windows 8 just a little longer.