Authentic Mexican is what the OP is asking. Most of you guys are posting tex mex, and to even suggest tijuana flats? I'm so sorry OP. But to answer your question, go to either
El Tapatio - 1804 W Vine St. Kissimmee, FL 34741
or Poblanos Mexican - 4747 W Irlo Bronson Memorial Hwy, Kissimmee, FL 34746
I rate Poblanos better than El Tapatio especially for tacos, but I just wanted to give another suggestion alongside Poblanos.
Here is Poblanos menu