Someone else asked the same question here at AMD Forum: AMD ThreadRipper mounting bracket replacement?
As I commented on that thread, Open a AMD EMAIL Support ticket and see if they can guide you on where to get another Orange Carrier Frame for your Ryzen.
Better yet, open a AMD WARRANTY REQUEST FORM and see if AMD will send you a replacement under warranty; https://support.amd.com/en-us/warranty/rma
Someone else I believe was able to find a source that sold them separately but I couldn't. It seems like the Carrier frame is part of the CPU as a whole and not sold separately.