The interesting thing about waterfalls is that the lead up doesn't have to be straight - Only the immediate tiles next to it have to be alligned. Here is a quick mock up of a waterfall scene which shows how mountains can retain a 'natural' look and still have the ability to have a waterfall.
As for the tiles, hover over your tileset in A-Map and look at the bottom-left corner of the program, it should state: "Block: Y, Offset: 0xZ". Just scroll over the tiles until you find the numbers you want and everything should be fine. The tiles you are looking for will have the wrong type of water underneath them (the light blue and flat one), but I'm sure you can make a new tile with the same attributes but with the correct water :3
Good luck and happy mapping! :D