My guess is that anyone's impression or scan is their personal health information. While I guess you could google this type of file and try to find one, I don't think you should expect any practicing dentist to give you a file of their own patients since its a violation of the patient's privacy. You could ask a dentist to scan your teeth and give you your own impression in STL format.
I understand what u said and i believe that its a personal private information which a DR should keep it safe but its a research situation and i really need to reach a model that i dont have it in my country .
the main problem is that the technology of digital scanning is not here yet and we cant have a 3d model in computer,i hope u can help me trough this problem and ask your patient to help me for university researchers.
btw thx for your replay