This might work in all similar situations, but an easy place to do this is World 1-1. Near the beginning of the level, there will be two Goombas walking down from a high platform onto another platform with a Question Block in the middle. Scroll the lower platform almost all the way to the left end of the screen. With perfect timing you can run back and knock the left-most block from below as the last Goomba walks over it. This will turn the Goomba into an upside-down green Koopa shell as he dies! If you knocked the Mushroom out of the Question Block as you ran by the first time, the shell will be red.

Another good place to pull this off is World 6-1, in the area with the hidden 1-Up Mushroom (see screenshots 3 through 5). If you're in the right spot, you can consistently turn Spinys into Koopa shells. Since Spinys survive knocks from below, the Koopa shells will also survive, and will turn into live Koopa Troopas on the ground.