Got my locking cap at O'Rielly Auto Parts. Left it on the pump in the Ozarks on our way to Wisconsin. Stopped at the first OReilly's we could find and got another.
Stopped at the station a week or so later on way home and the guy said he was about getting ready to throw it away. Asked him if I needed to show him the key that worked, he said no, no one but the owner would know to ask about it. Now I have an extra for the next time I do the same stupid stunt.
Went to NAPA and they didn't have the exact fuel cap, but had something that barely worked, so I bought it.
A week later I was in Walmart for something unrelated. Looked up, and what-a-ya know, there it was. The exact replacement fuel cap. A whopping $4.95 as I recall.