Mix your own. To learn about paint mixing (to get the right color) check out a book for artists, and invest in a color wheel (they are usually not expensive).
There are the purists who worry about duplicating the shade the prototype had the day it left the factory. Others of us like to depict a subject after it has been in service awhile. Unless it is a recent subject, older paints weathered very rapidly. Cars can be kept in garages, but most military subjects are affected by UV rays from the sun and oxygen in the air and the color changes depending on their exact environment. So if you depict weathered subjects, the paint mixing does not need to be absolutely precise.
As a way to get started in paint mixing, try a little weathered black. No real surface looks absolutely black- certainly not tires or things like that. Sure, you can by a weathered black color at the hobby shop, but try mixing your own. Add just a drop or two of white or medium grey, maybe a very small drop of brown, to a flat black.
Best to stay with the same brand and type, however. Do not mix colors from multiple brands or types.