Once you have your ISO file:
For Windows 10, Microsoft offers a Media Creation Tool. Although not directly available for Windows 8.1, this tool can help you create installation media for the version you have a key for. download windows 8.1 disc image (iso file)
Historically, Digital River has been a partner with Microsoft for distributing Windows versions. You might find Windows 8.1 through their website but be prepared to provide a product key. Once you have your ISO file: For Windows