Now that it has been established that K & S make hex brass section, do what I've been doing for years, just use this stuff to turn the bolts from. Before parting the bolt from the parent metal, why not use a die and now thread the bolt as well.
To make the nuts is just as easy, just drill and tap the hex section whilst it is in the lathe chuck and then part it off, to whatever thickness nut you require.
This way you can then test fit each and every piece to the model and it can be removed easily for a rework if required.
I make hundreds of bolts and nuts for each model that I make, even made up a special machine to make the nuts real quick, still use the lathe for the bolts tho'.
OOoops, forgot to say that you have to go look thru the SPECIAL SHAPES to get to the hexs bars.