Any department store will have Lego, or if you can find a toy store, they usually have a good selection and sometimes older sets too. As for the department stores, you should avoid Wal-mart as they are usually running low on Lego. You should visit Zellers or Toys R Us if you can find one
Don't get the Crystal King The Rock Wrecker looks promising to me, and so does Cave Crusher. It depends on the price of the Cave Crusher to decide which one though.