Where can I find a 5/16" end cap for a faucet diverter?
The problem here was using the right search terms. Based on Tester101's comment I tried some additional terms, especially "flare". Although I did not originally want a flare cap, it helped improve the search results enormously so I was able to find the right kind of suppies and narrow down to something that would work. Ultimately I got this:

5/16 Brass SAE 45 Degree Flare Tube Fitting Cap - 5-Pack