You could make one as simply as cutting a disc from a stick, and burning or carving the pentacle into the wood.
Or you could bend some wire into shape.
Or, if you want to be a bit fancy, get a sheet of brass or bronze, plan out your pendant, drill a few holes to pass a jeweler's saw through, and cut it out.
You'll also have options for sweating (a method of soldering) layers together for a more complex project.
If you have the materials (or can make/assemble/acquire them), you could melt the metal, and pour it into a cast.
Basically, your imagination is the limit.