If you have Win8 Pro you should be allowed to [b=http://www.microsoft.com/oem/en/licensing/sblicensing/pages/downgrade_rights.aspx]downgrade[/b] it to the previous version. Most likely you have the basic edition, though.
Getting a Windows 7 license is getting hard since Microsoft don't sell it anymore. You may find it at some store with a little bit of luck. Another way is to find someone who has bought a three computers license for Windows7 upgrade, only using it for one or two. I suppose reusing the code and sticker from an abandoned Windows7 computer would be against the M$ EULA, although the idea of using one oem license on one machine would be fulfilled in that case, too. Anyway, a legal and functional license code for Win7 is what you need. Don't use a pirated or hacked one, they tend to get unsupported. Get a real one.
Getting an installation media for a valid license is no problem, even borrowing or copying one should be totally legal. At least at one point Microsoft even suggested using a copy of the original CD to avoid it getting damaged.
