Windows 7 Enterprise is a volume licensing product available companies that do large deployments of Windows. If you have 5 machines or more with a qualifying premium business edition of Windows such as Windows XP Professional full retail/OEM, Windows Vista
Business, Ultimate full retail/OEM, Windows 7 Professional,Ultimate full retail/OEM, then you qualify.
Also, to get Windows 7 Enterprise or Windows 8 Enterprise specifically, you need the Enterprise Agreement with Software Assurance.
(In final analysis, its expensive for an individual).
If you need the features of Enterprise, purchase Windows 7 Ultimate which is basically a retail version of Enterprise without the licensing restrictions.
Where can I still get Windows 7?
Full version - Microsoft
Windows 7 Ultimate
Upgrade version - Microsoft
Windows 7 Ultimate Upgrade