I assume you're speaking of replacing the BALLS.
If so, it's pretty simple. You need to match the diameter, typically 3/16" though may be different.
They are always steel, regardless of the frame, which they don't touch anyway.
If you prefer a retainer, it's brand and model specific. However most experienced mechanics prefer loose balls, and many believe that headset life is better that way.
Determine the size, then shop for grade 25 balls. That's much better than needed, but a super cheap luxury, so why skimp.