Fat grafting with low BMI
Ophthalmologist are trained in the ocular area only. It sounds like the ophthalmologist you saw is not comfortable with liposuction of fat in more uncommon areas, which I frankly commend.
You are correct that filling of the superior sulcus usually takes less than 1cc of "good fat" per side. To do this, however, more fat does have to be harvested, but regardless, unless you are exceptionally skinny, enough fat should be able to be found. See a board certified plastic surgeon who is experienced with periocular fat grafting.
The grafted fat pearls are a largely discredited method.
One can not reliably fill an upper eyelid sulcus using grafted fat pearls even though there is an old website on oculoplastic surgery that seems to promise that this method works. I can assure you that this information is far less than meets the eye. Unfortunately the upper eyelid is the ultimate set of guide planes. When a surgeon opens up these plans and inserts your own fat into the sulcus, this fat is not like native fat. This fat causing scaring. Rather than the beautiful eyelids we see in nature, these eyelid are irregular, scared and don't move well. I strongly recommend that you do not allow anyone to place fat pearls in your upper eyelid no matter what you have read on the internet. On the other hand, one does not have a hollow upper eyelid sulcus because they have a low BMI. That is nonsense. Most likely you have an early levator dehiscence. A photograph would of course immediately answer this question. If that is the case, repairing the dehiscence brings orbit fat forward and also allow the eyebrows to relax down helping fill the upper eyelid sulcus without all the scaring that will be created by grafted fat.
Fat grafting of the upper eyelid
FAt can be obtained from anywhere in the body. The best places for harvest include the thighs, abdomen, and even buttocks. Although little fat is necessary, more fat is harvested in order to obtain purified fat cells. Fat is an excellent filler for the upper eyelid.
Superior eyelid sulcus hollowness filler injection
You should consider filler injection instead of fat, especially if not enough supply is available. Filler injection in upper eyelids is very effective. See following video and link.