Hair under chin as donor hair for facial hair transplant?

I am in my mid 20's and my hair is starting to thin, but I am also unsatisfied with my patchy facial hair. I do not want to use valuable scalp donor hair as I may need it in the future. From what I understand, body hair is less likely to look natural or stay permanently, and using neck hair can leave visible scarring. What about the hair under the chin? How many donor grafts can usually be found there, and how visible is the scarring?