best practice is to change the entire joint cos chances are its got dust/grit/water in it by now which means it wears out a lot quicker than normal,the cover is after all there in the first place to keep lubricants etc in and all the above out.parts factors do supply replacement balljoint covers which ive seen used with varying degrees of success depending on the vehicle and abilities concerned.cant tell you if its a complete arm/alignment etc or what else is required.knowledge of what vehicle it actually is and indeed which joints would be required for that.
