Not a cd. It's so big it only fits on a DVD. You can buy those in a shop or a webshop. But you can also download it from microsoft and burn it to DVD yourself. Your choice.
Details might depend on what version of Windows Vista you have (varies from Basic to Ultimate) and what version of Windows 7 you want. And if the Windows Vista is 32 or 64 and if you want the 32 or 64 bit version of Windows 7.
http://windows.microsoft.com/en-us/windows7/products/compare#T1=tab01 is a good start. It shows the boxes you can buy.