this is not the cheapest they have been, but very reasonable price for something that is pretty hard to find:
https://www.ebay.com/itm/352441604068
Supermicro Mounting Rack Rails Kit For SC748 4U Chassis MCP-290-00001-00 | eBay
if you have an older style Supermicro 4U server and have been looking for the proper rack rails... hope this helps!
https://www.ebay.com/itm/352441604068
Supermicro Mounting Rack Rails Kit For SC748 4U Chassis MCP-290-00001-00 | eBay
if you have an older style Supermicro 4U server and have been looking for the proper rack rails... hope this helps!