This stage is Sprawling Savanna (5-4). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. We will also note a secret 1 UP.
Game Play Walkthrough Video
Green Star 1
You will need the Cat Mario to grab the star easily for this one. There are two ways that you can take to grab the star, first is to climb from the bottom, however you may hit some invisible blocks that require you to slide back down and climb back up. Or you can use the cannon to get all the way to the top, then slide down into the hole with the star.
Green Star 2
After the cannon shots, you will find some rabbits in the open area. Go after the one with the green glow to claim your second green star.
Green Star 3
The last green star is obtained by collecting all the green coins at the spinning platform.
The stamp is located inside an ant hole at the huge flat area. Look for a stream of ants carrying coins into the hole, jump into the hole and get the Stamp from a giant ant.