Just download the High Sierra Patcher. Not to use it to shoehorn it to an unsupported machine, but you can use it to download a current (10.13.6) High Sierra installer. I used it a couple weeks ago for this very reason. All you will need to apply after is the 007 security update, safari update, and a couple small iTunes patches. It's a pure installer from Apple also, not a patched or sketchy one in any manner.
Thanks! I was able to install High Sierra this morning after deleting the original installer, then it properly re-appeared in the App store...all good now..