You can buy a copy on line here:http://www.amazon.com/microsoft-windows-7-8-software/b?ie=UTF8&node=1286119011
As to how... reinstall windows 8 and during the prep phase wipe and delete the partitions of your hard drive. As soon as that is finished, exit the installer. Replace your eight disk with a seven disk and install normally.
I'd get home premium. If you run into issues of the, "I can't do this with home premium" kind, then upgrade to professional. Upgrading home is cheaper than getting pro right off.