I did find one at ACE. Made me realize if something so common and simple as a cover plate can't be found then the issue is indeed very deep.
I am finishing up an off-grid solar installation. I have been unable to get the electrical parts that I need to do the job easily. Plastic single gang switch / receptacle boxes do not exist. The metal equivalents are a bit more work all around but even the metal boxes are diminishing. I have spent about $300 buying up what I think I might need... I am a retired electrician and I try to keep a stock of simple common pieces, so those are parts I am stocking up on a little bit also - doing occasional service calls for friends.
But almost none of these parts are the sort I used to keep a stock of; they are the best substitute available. sigh