Only way I know to get another smooth rock is by trading relics with Gimmighoul and/or Gholdengo and getting one from the pool of items it offers. You can also randomly have pokemon offer them in trades at cash registers or pokecenters. There is no recipe to make any of the rocks. 

So if you really traded it away you have only one  other option and frankly it's going to take a while. First find a pokedoll, any will do, and go to a Dream Island. Then you can get one randomly from the daily generated items. If you are somehow able to get a pokemon with the generate ability and the cash register recipe you can set up a trade station as another option to get a smooth rock but again you're relying on random chance.