Everything in a carb has to be corrosion resistant. Like it won't rust. Brass, bronze, copper, stainless steel, aluminum. SS ball or AL ball is OK. "Steel", like carbon steel ball..NO! If it's magnetic, you can't use it.
If you don't mind me saying: When you rebuild a carb, you buy a rebuild kit. You don't by balls and other parts at a hardware store. If you buy the correct kit, use the new needle valve provided. It will also have the ball(s) you need any many other parts that MUST be replaced. Everybody builds their carbs at home. It's good you are doing it. Just do it with a kit. Possibly
this one.
