I hired one from 12 weeks with both of my pregnancies
I've had two missed miscarriages so early pregnancy for me is pretty stressful. I've found the doppler has saved my sanity until I can feel movement - and even after that if baby was having a quiet day it's been good to be able to check on them.
Never had any problems whatsoever finding the heartbeat - as long as you follow the directions by 15 weeks you shouldn't either.
I hired mine from First Beats - you can google it to get their website.