It doesn't have to be the root or the root bark.
If the branches of your barberry (Berberis spp.) bushes have bright yellow bark it'll work as well as the root or root bark.
A plus is, you don't have to dig the bush up. Which means that one or two largish bushes will supply an ample amount of branch bark, as opposed to the "gotta plant half a dozen or so, so I can dig one up every couple years".
A minus is, you'll have to cut the very sharp spines along the branches before you can start stripping the bark off. Or you could of course leave them on, but then you'd prick yourself every time you used some barberry branch bark.
Pic: Bark of Berberis branches. Some of the branches we peeled day before yesterday were a very deep yellow almost down to the soft center bits. Those yellow branches I sliced up and added to the peeled bark, to dry.