As Piglover stated above, I've not seen a reasonable priced used ones. Here is the part# if you want to try your luck
Cable Management Arm MCP-290-00073-0N - Supermicro Cable Management Arm for 2U, 3U and 4U chassis
"Q: What is the purpose of MCP-290-00073-0N (Supermicro Cable Management Arm for 2U, 3U and 4U chassis (Extendable Length: 70mm to 830mm)); and can it be installed on SC846E16-R920 chassis?
A: It's not a replacement to rail kit; and the purpose is to manage the cable neatly on the back of the chassis.
It does not support SC846 chassis; only the following models are supported: SC219, SC829, SC828, SC835, SC836, SC936, SC743, SC745."