You can try this link first :
http://windows.microsoft.com/en-us/windows-8/upgrade-product-key-only
You can use the above link to download both WIN 8 and WIN 8.1
Click the "Install Windows 8" button. Enter your Windows 8 product key and the setup program will start downloading the files for Windows 8. Close the setup program.
Go back to the page above again and then click the "Install Windows 8.1" button. Run the setup program for 8.1 and it'll start downloading the 8.1 ISO and skip the step where you have to enter a 8.1 product key.
Once the 8.1 files are downloaded, choose to create install media. That's when the program will create the Windows 8.1 ISO. Pop in a writable DVD and burn a copy or use the Windows 7 USB/DVD download tool to create a bootable USB stick from the ISO.
You can create Bootable DVD/USB from the tool linked down :
http://www.microsoftstore.com/store/msusa/html/pbPage.Help_Win7_usbdvd_dwnTool
If the above procedure is much of a hassle for you, then you can download the ISO from this link, later you can make Bootable media from the above link :
http://forums.mydigitallife.info/threads/47732-The-Windows-8-1-(-amp-related)-Repository
Note: You will need a torrent client to download from above link : its free : http://www.utorrent.com/
P.S. Always install the non-win OS first, then win OS. Good luck.