The part you're looking for is probably called the "Right Speaker Cup". The part number will vary based on the color of your AirPods Max: 661-18238 (Space-Gray) and 661-18241 (Sky Blue).
In terms of where to purchase, your best bet is going to be either eBay for a "donor" device that you can harvest the part from, or from [link|https://www.joesge.com/products/apple-airpods-max-headphones-spare-replacement-repair-parts?variant=39953735712814|this|new_window=true] website.