It might depend on which model of barcon.
The original ones have only a small shallow seat for the cable outer - 4mm dia and about 2mm deep.
They rely on the outer being held in position by whatever bar covering you use; done properly this works well enough.
So if you can't find the correct step-down ferrules it shouldn't be a big issue[1].

A useful trick I devised is to get a thin plastic tube, just larger than your outer, and wrap *that* - with the outer within.
I found the local hobby shop sold length of music wire in exactly the right size tubes.
This means you can change outers without destroying your bar wrap job.

[1] If you can't but absolutely must have proper-sized ferrules let me know, I made mine, easy to do more.