Where is the Greatest Amount of Usable Fat Located for a BBL?

The "Best Place" to have fat removed from, is where you want fat removed from.  It is your body.  I have performed the Brazilian Butt Lift for about 15 years now and never liposuctioned an area just for the purpose of obtaining fat. I will only liposuction an area if you want that area made smaller or shapelier.  If you want your thighs made smaller, then ask your surgeon to liposuction your thighs.  I almost always liposuction a woman's waistline (Abdomen, waist and flank areas) as part of the BBL procedure because it is the slimming of your waistline along with the enlargement of your buttocks that gives such a dramatic and beautiful hourglass shape! 
As far as the "stubborn fat" theory.  It is a theory.  I'm still not a fan of that theory.  I have used fat from all over the body for buttocks enhancement and have only found one area that seems to lead to better fat survival than others.  But I still do not recommend taking fat from there unless you want that area made smaller.
Good Luck
Dr M

we could take fat from inner thighs, belly, love hands,posterior arms, inner knees,and that of course would be enought to realize it unless you be under your normal average age, height, weight then in that case it would be recommended to get it from macrolane.

nosotros podriamos tomar la grasa de  la parte interna de los muslos,de las rodillas, del abdomen  de las llantitas,y eso seria suficiente, por supuesto siempre y cuando no estuvieses por debajo de tu peso normal de  acuerdo a tu estatura, edad y peso