Ah. I did find a list of keys that I can use to download (but not activate), but I can't really link to them.
To do what you want in an EULA consistent way, you need to pay the price of admission--which means that you have to buy a second product key. In this regard, there is no legitimate way to get an ISO without having or obtaining a valid product key--not including an embedded OEM product key. In this regard, I am ignoring the possibility of creating an ISO from purchased retail software--which would automatically come with a product key in the first place. Note that I am not going debate whether one should or should not be able to do what you want without buying a second product despite existing EULA's an OEM agreements.
So, you have, IMO, two at least semi-reasonable alternatives (i.e., ignoring getting an ISO--which you also could do, but the issues would not go away). First, you could purchase a retail Win8 Pro upgrade software set (not going to deal with plain Win8--why bother?). This set comes with two discs (32-bit and 64-bit), and the 64-bit disc will have UEFI support. Given this set, you can install anyway you want. But, there is a possible question about the EULA consistency of using such software to reinstall even the same version of Win8 on an OEM computer (unless you forfeit a key from other qualifying software). I'm not going to get into all the issues that might be raised here.
The second alternative is to purchase
MS system builder software and install under the personal-use license (you can get at least 64-bit pro with UEFI). This software is full-install software. Thus, this disc can be used to install any way you want and over anything you want with no qualms whatsoever (unless there is some specific prohibition/stipulation from your OEM). This alternative is the one I would choose. In this regard, I would gladly use the software to obliterate an OEM installation. I like this alternative because it would allow me to get as far as possible from being under the thumb or either MS or an OEM.
Confusion can occur when reading the EULA pertaining to this software/license. If interested, then see this thread:
Win8 OEM System Builder License. My conclusion, based on the discussion in the thread, is that the correct operational interpretation is that anyone can use this software and license.
Here is what I think is allowed to anyone buying such software and installing under the personal-use license. First, you can have one installation at a time on one owned computer. Your installation can be moved from one owned computer to another owned computer an unlimited number of times. Any owned computer can be changed in any way an unlimited number of times without affecting your installation rights. You can transfer the software/license combo to another individual if you are willing to forfeit the right to use the software/license yourself.