You can buy the Win 8 Pro download now, download as an iso, burn it to a DVD and you will get a product key that you can use to install and activate it at a later date.
There is also a free download of the Windows Media Center that requires you have Win 8 Pro installed. Unlike the Win 8 download, the media center product key will only work to install media center through Jan 31, 2013. So if you plan to watch DVD's on your Win 8 machine, you will need to install both Win 8 Pro and Windows Media Center before the end of the month. Or you can buy Media Center later, or use another DVD player.