GOG pulled a rare dick move delisting the original so now I'm forced to spend more buying the Demaster when the original 2 Gold went for as low as a dollar.
It was my fault for not buying it back then but now I'd like to know if there are places to get a GOG key for as cheap as possible, hopefully not having to dish out 4 times its original lowest price here.

Just to be clear, it wasn't GOG's decision to delist the original, it was the publisher, THQ Nordic. They did the same elsewhere, as well.
Sadly, I don't know much about key resellers, I don't trust and I don't use them, that's why I'm spending money on official stores. Well, at least when they provide the suitable circumstances for it. But if a publisher pulls a scummy move on me, I don't feel remorse when I pull a scummy move on them. I'm sure you can find a place where you can get it for a really good price.