I had a new air condition unit put in 15 months ago, It stop cooling so two parts had to be replaced in the last 2 months. Now the unit is leaking water through my ceiling and I was told the drain pan that goes under the air filter is not there. What should I do or who do I call ac or plumber?

Call the AC guys back out. Sounds like they didn't put in a the overflow pan. When a unit is installed in the attic it absolutely has to have an over flow pan. They should have to pay for ceiling repair.