Larry Hartke is an active member here, he'll probably chime in.
If my memory serves me correctly though, what they use in 'em is what people refer as "RS" knobs, ie. the most basic splined small knob with a removable color coded hat.
Walk into Your local RS or eqvivalent, you should find something very similar if not the same in the knob section.
Again, if my memory serves me correctly, it's been a while since I had a Hartke product, the potentiometers are pretty standard ones so You shouldn't have any problems in getting those replaced.
Unless of course You smashed one of the sliders or misplaced any of the slider knobs, then you're pretty much SOL unless the Hartke service sells spare parts.