Ouch!
I do exhaust staining using the same paints that I paint the rest of the aircraft in. I typically use greys, browns reds and black to build up something appropriate.
I have never felt the need to buy anything that is not in my paint draw already, I've got better things to spend my money on, and anyway, no one-off product can replicate the subtlety and complexity of the real staining.
Staining can vary from the almost white seen on aircraft like Lancaster bombers, running very lean mixtures of highly leaded fuel for hours on end, to the predominantly black seen on Luftwaffe fighters doing almost the exact opposite. There is just about everything in between.
Love 'im or hate 'im, Brett Green does some really good exhaust staining. I have adopted a system very similar to his since I first saw it many years ago.