Not quite sure what you mean by "Mounting Plate". Are you talking about the metal plate behind the motherboard that is used to mount the CPU Cooler?
If so, that normally comes with the Motherboard when you purchase it. Many high end CPU Coolers come with their own motherboard Mounting Plates to replace the one that came with the original Motherboard.
If this is the Mounting Plate you are talking about, then you need to Contact your Motherboard's Support site and see if they will send you another one.
If you are talking about the Orange plastic mounting plate for the Ryzen CPU/APU, I believe you can get a replacement from an Online AMD Warranty Request.
EDIT: I see that the Ryzen 5 2400g comes bundled with the AMD Wraith Stealth Cooler (no LED). Since this CPU Cooler came with the Ryzen bundled it should be under Warranty. I would open a Online Warranty Request ticket and see if AMD might be able to replace the missing Wraith Motherboard mounting plate from here: AMD Warranty Request Form | AMD . Explain to them what happened. They may say to go to Retailer that you returned the Ryzen under Warranty but if the Retailer won't replace the CPU Cooler that you sent in maybe AMD Support under Warranty might.