using $http in angular.js factory

You need to add a callback reference in your getSource() function of your service:

app.factory('autoCompleteDataService', ['$http', function($http) {
   return {
       getSource: function(callback) {
          var url = '...';
          $http.get(url).success(function(data) {
             callback(data);
          }
       }
   }
}]);

You could also use $http.jsonp, if your server returns json. Don't forget the JSON_CALLBACK parameter then.

In you directive you need to add the callback function itself:

...
autoCompleteDataService.getSource(function(data) {
   elem.autocomplete({
         source: data
         minLength: 2
   });    
});