Couple of ways to do this, theres a a guy named Grusderk, who has a method that workswhenyou open your mouth it moves the mandibles. the problemwith Grusderk is his masks are poorly made,
Complaint Topic and he's not currently making them right now.
Discussion of the Grusderk System
Theres also r1a2s3, who makes a system similar to gruderks system, but apparently is a bit more reliable.
r1a2s3's method(with Grusderk comparison
There isalso a guy who sells animatronics, but, those will run you lots and lots of cash, and can require a different style of mask,and couldneed a second operator.
Hope these links help, and good luck!