こういうやつ？
https://www.ebay.it/itm/65-IVA-IBM-Lenovo-00KA063-PCIe-Riser-1-1x-ML2-x16-CPU0-for-x3550-M5-00KF625-/114255175707

普通にconnectx4買った方が良さそうな感じだな
https://www.ebay.com/itm/Mellanox-MCX413A-GCAT-ConnectX-4-Lx-EN-Card-50GbE-Single-ports-PCIx8/184299049560