Its in the cave leading to blackthorn city, you need to skate on ice to get to it. A man dropped it he is a little farther in the cave but he will let you keep it!
How To Get Waterfall In Soul Silver?
Beat the 8 gym leader after go to the cave behind the gym with a pokemon that knows whirlpool get to the bk of the shrine and tlk to the old man hell give ask question answer them with the answer that sound the kindest if you succed hell give you the tm waterfall