Have you tried online? JC Whitney and eBay are pretty regular sources of all kinds of "my store OUGHT to carry this, but I can't find it" stuff.
Here's a dozen 10psi caps on eBay you can match yours against:
10psi cap | eBay
There's a hexagonal one 4 items down that looks like a truck part and another one a few spots further. Maybe the same as yours? If not, keep scrolling.
If you find the right part, why not buy two? They don't really fail much but they're super cheap and then you'll have the spare. And if you CAN'T find it, any decent welding shop can weld on a new neck that takes a different kind of cap. It's kind of a simple job if you don't mind emptying and bringing them the part - I had one done just to shorten the neck and the guy only asked $50, I think.