The dye comes in powder form so you can dilute it way down and layer it up for very good control.
The old timers would use it like Photoshop to add clouds to an empty sky or cigarette smoke to a
portrait, back when that nasty habit was fashionable yet they didn't want real smoke in the studio.
I'd just post an ad on this forum etc looking for it, or hunt Fleabay. It was a standard Kodak product
for decades and is more versatile and less messy than shading pencil. You could probably buy bulk
dye and make your own, but a pound of the stuff would suffice for a hundred darkrooms.