Where to find the ssd pcie to my iMac 27" 5K 2017

Hi, I have imac 27 5K 2017 with Fusion Drive 1TB (24GB SSD PCIe + 1TB HDD). I would like to buy a 256GB SSD PCIe, I know that only I can buy the original Apple but I do not know which model to choose? I only want 256GB no more. In place of the HDD will be SSD SATA Samsung 1TB EVO 860 - I already have a drive. Please help me choose which drive I was looking for on eBay and I could not find models that would fit, only MacBook Air and Pro, not iMac 27. "My computer has a PCIe 3.0 x4 or x2 connector?