For small revolvers such as the 638 I prefer a pocket holster. Since I can work with leather, I make my own. But the one dahermit (above) shows is excellent for the money.
I might add, you will have to wear pants that don't fit tight. I wear looser fitting jeans and Dockers with pleats. And you must dedicate a pocket to the pistol. You can't carry change or keys in the same pocket. Only the gun.
Hope this helps.
Gene