This depends on which game you're talking about.
If it's Pikmin 1, then in the forest of hope, go right from your base's first entrance (the other being the rock wall you can't break down) and break down the white wall you should come across. Break it down and you should see it. Mind the bulborbs.
If it's Pikmin 2, start the long adventure at the second area (I forgot the name. Sue me.) find another half of the globe somewhere. (You're gonna need white pikmin to get it) The next day, there is another area available called the perplexing pool. Go there. The ship should make note of the yellow pikmin. Then find your way over and get them. If you think you need blue pikmin, you don't. Just make sure you bring red pikmin with you because there is a fiery bulborb gaurding the entrance. Then get white pikmin to break down the following poisonous wall! Then, they're yours.
The Onion will sprout 1 yellow pikmin when there all dead
After collecting the 2nd half of the globe treasure found in the Awakening Wood, go to the Perplexing Pool. Walk forward to find a cutscene showing the Yellow Pikmin. Then continue to walk past the Yellow Wollywog, to a set of scales. Throw all but 1 pikmin up there. Throw the last pikmin, while standing on the lower scale, on to the higher one to be lifted up. Once there, kill the Fiery Bulblax, and break down the poison gate. Walk to the pikmin and whistle to them and Yellow Pikmin are all yours.
The main characters are Olimar and the red, yellow, and blue Pikmin.
You just have to find it. It could appear anywhere.
Once you find a type of Pikmin, their Onion comes with them. If you find blue Pikmin, for example, you automatically have the blue Onion.
No. The only Bulborbs in Pikmin 3 are the Red and Orange varieties.
In the first game, the yellow Onion is in the second area, the Forest of Hope. Take enough Red Pikmin to defeat the Bulborb north of where you start out. Then, knock down the wall and kill the Bulborbs, dismiss the red Pikmin, and go north. You should see a darkish yellow colored Onion in the ground. Approach it, and it will turn bright yellow and rise out of the ground, and produce a single Yellow Pikmin sprout which you can then multiply. In the second game, the yellow Onion is visible immediately upon entering the third area, the Perplexing Pool, along with a few wild Yellow Pikmin. You will need Red and White Pikmin to reach them. First, head around the side of the stump where you see the Onion, where you'll find a scale puzzle. Once you get passed that, you'll see that the only way to where the Onion is has a wall raised with poison generators beneath it, and a Fiery Bulblax blocking the way to that wall. Use your Red Pikmin to kill it, and then use your White Pikmin to break down the gate and disable the poison generators. After that, all you'll need to do is whistle the wild Yellows. In the third game, Brittany finds the yellow Onion immediately upon entering the third area, the Distant Tundra. She'll wind up in a cave where she'll find two Yellow Pikmin trying to light a light bulb, and three Pikmin sprouts in the ground. With all five, the light bulb will light, and the Onion will bloom and rise from the ground. Within the cave, there are enough pellets to generate twenty more Yellow Pikmin, for a maximum total of 25; you will need at least twenty to open the way out of the cave.
Ask your spelling teacher
Five yellow Pikmin can be found in the giant stump in the middle of the Perplexing Pool. They are not easy to receive, however. A slight puzzle is located to the left of the stump that you must first solve, along with a hoard of enemies.
In pikmin 1, you must get to the Forest of Hope, then go right, destroy the wall at the end then approach the onion in the ground, and in pikmin 2, you need to get to the Perplexing Pool, then go left on the white trail, then go right at the end, then take your red pikmin and white pikmin and kill the fiery Bulblax, then destroy the poison dirt wall, then whistle the yellow pikmin off the tree.
Olimar named the Pikmin such because of their similarity to the red, blue, and yellow pikpik carrots in his favorite soup.
Bomb rocks can be picked up and used by yellow Pikmin to destroy stone walls.