gender reveal?

Alison • 37-married 3.29.15-in nursing school-11 yr old stepson-finally pregnant with our first baby together!
How early can you find out the gender of your baby? Will those private 3D sonogram people tell you earlier if you pay for it? Wanting to do the Gender reveal party during Easter when my stepson is in town but I will be cutting it close if I don't find out til my 18 week appt.