Believe it or not, I have better luck finding brass fittings like these at True Value hardware stores, with about 1/4 the square footage of the big box stores like Home Depot.
I did the same thing to my boost gauge. Went to Home D and found less than nothing. Even went to several industrial supply and air fitting stores, and still nothing.
I finally found it at a larger True Value store, but it wasn't in with the air fittings - it was in the plumbing section.