If you have both the PSU and the motherboard already what is the problem exactly?
Just the power cable?
Because it's something like THIS: http://mashup.bbchaoyang.com/2018/07/27/genuine-dell-poweredge-6800-6950-9138r-pdu-power-to-server-32amp-c19-to-c20-3-prong-black-power-cable-cord-plug-connector-extension-compatible-part-numbers-9138r-09138r/
I'm not sure HOW you intend to use this but it may not be worth spending money on if you end up needing to buy anything expensive. You have the CPU's already etc?