It's long been a major pet peeve of mine that Microsoft has made it such a challenge to procure a legitimate ISO image of its OSes. I've felt like the company should have no problem offering them in an easy-to-find spot on its website, because after all, it's not like they can be taken proper advantage of without a legal key. Sometimes, people simply lose the disc or ISO they had, and so it shouldn't be such a challenge to get a replacement.
Well, with a new feature on its website, you are now able to get that replacement ISO. However, it's behind a bit of protection: you'll need to provide your legal product code, and then the language, in order to go through to the download page. If you've somehow lost your key but are still using the OS that it's tied to, you can retrieve it through a tool like the Magical Jelly Bean (an application I've used for many years and has saved me a time or two).
In quick tests, not a single valid Windows 7 product key I tested with this feature worked. One spit back an error that I had selected the wrong language (I didn't - I've become quite accustomed to the English language), and the others gave me oddball errors that just told me to contact Microsoft. I even tested a couple of "N" keys, and none of those worked, either. All of the keys I have either came from Microsoft itself, or through MSDN.
I hope that you'll have better luck than I did. If you do get an ISO downloaded, you can either burn the image to a DVD as usual, or create a USB-based installer using Microsoft's own USB Tool, or third-party tools such as UNetbootin or Rufus. For advanced users, Microsoft's command-line utility diskpart could also be a good option (but that requires reading up).
Hopefully this is just the start. Where are those Windows 8 replacement ISOs, Microsoft?