10-24 x3/8 x1/8
Presumably you mean 10-24 thread, 3/8 across the flats, 1/8 thick; that seems to be the standard size. As others have asked, square or hex, what material, etc? They should be available at your nearest Ace hardware.
In my experience, commercial nuts for the smaller threads have only a couple of threads in them so if you need some strength, you would be better off making them yourself.