I don't understand why you won't use rattle cans, but you can always buy the Afrika mustard and decant it for airbrush use. The FSM website has a fine video on decanting spray cans.

EDIT: One more thing, I don't know what I'd do without one particular rattle can, and that's testors dulcote. It blends and smooths and gets rid of that shiny toy-like look. So even if you absolutely refuse to use rattle cans of any type, dulcote should be the exception to the rule.