This is a late reply, don't know if you found a replacement door...
I went round and round with the Apple store. My door was damaged when I upgraded my ram on my new iMac 27". Apple needs to 1st see your computer to order the part, they order the part & then you bring it back in and they install it. That is if it is a brand new computer. The part is not available any other way, you need to pay for the part, it is about $35. I went thru that whole thing only to find out that now the Apple stores are closed becauses of Covid, they told me they are not going to be ordering or replacing the access ram door. So I guess I am up Sh**Creek & you too? BTW, they are no spare parts sitting around at the Apple stores, at least an Ram Access Door