Modern brake ferrules and derailer ferrules have the same ~6mm OD, but the ID is generally 4mm for shift ferrules and ~5mm for brake ferrules.

These alloy ferrules have the internal plastic seating ring for the sharp shift cable housing strands to embed into, which equalizes the compressive force among the strands, and which helps prevent any strands from migrating inward into the cable's path when the housing gets bent at the ferrule.
So it's best to have nice, sharply-cut strands at the ends of shift cable housing.

Often a bag of ferrules is sold with a roll of cable housing, or as mentioned the kits come with ferrules. They can be ordered separately.