How this adapter inside the bearing is called? How to find/make a replacement?

This is an ABEC7 bearing with a 12 mm hole and a 19 mm external diameter:

It has a tightly mounted adapter (the black part), which in turn has a 6mm hole and two external diameters: 12 mm (partially hidden in the bearing, partially visible) and 15.5 mm (fully visible). The adapter is required for a common 5 mm axle of two bearings. The wider parts are on the outside. The narrower parts are fixed into a common wheel.

Unfortunately, the adapter is made of aluminium and I would want one which is made of steel. Thus the questions:

1. What is the proper technical name of the adapter?
2. Can it be bought "off the shelf"?
3. If not, can it be easily ordered from a typical metal workshop?

EDIT: I was asked for a precise drawing and so I attach it. As seen, I need two versions, both with an increased internal diameter. I could adapt, however, an off-the-shelf part into any of these versions with a drill, apart from the 16/18 mm outer diameter, which is better but not necessary.