When searching for the 2015 Nightmare Megatron on ebay, also search for the 2012 Dark Energon Megatron, which tfwiki says is the same, but the official images looks like there might have been some minor colour changes (nothing too significant).
I'm not sure if the 2012 version is easier to find or cheaper, but I'm just noting it in case it is and you are okay with either version.