In cases where only a small amount of cartilage is needed, it can sometimes be taken from another part of the ear where it will not be noticed. In more complicated cases rib cartilage needs to be used.
If there is skin missing, then usually the skin which sits behind the ear can be lifted up and used to fill in the missing skin from the ear. If this is needed, then is usually means there will be a second operation needed to free the ear up and let it sit our from the side of the head again. This second operation is usually done 3 months later.
If your problem relates to a bad otoplasty (prominent ear correction), it can usually be fixed in a single operation as there should be enough skin present. The cartilage is reshaped and often requires a cartilage graft of some sort.