If you don’t have enough scalp hair then Beard hair is usually next up, you could go chest hair too, but bare in mind the yield will decrease.
Occasionally we see cases where other hair is taken with varying degrees of success, Umar was once the pioneer. These days Drs are mostly using Beard hair but added to mostly Scalp hair, I can’t remember where I saw it but somewhere is a table of the possible yield of each source, leg hair was the lowest.