If you have something very specific in mind but can't find anything that suits, you may think about commissioning something....though of course that can get quite expensive, depending on what artist you use. But otherwise you will either have to make do with something less than exact, or do a lot of customising work yourself on it to make it look how you want. Sometimes the person or character you are trying to recreate is too specific to use a generic or celebrity sculpt, so it's something to consider.
ETA: Even if you don't want to commission a complete headsculpt from scratch, you could consider having an existing one professionally repainted to look more like how you want.
ETA: Even if you don't want to commission a complete headsculpt from scratch, you could consider having an existing one professionally repainted to look more like how you want.