Can't you get a mixed paint to match a sample/specification from a decorators/art supplier? I have had some particular shades mixed up when it was more economical than buying several paints and mixing the same shade by hand. (For application by brush and sponge, and as a wash, not for an airbrush as I don't have one... and I'd varnish before/after the colour coats to make an unknown quality behave as expected on a model, but it might be a possible way of getting any colour you need without the search being long and fruitless.