Product availability and price are subject to change
Why do you need/want a Peanut? Because it offers dynamic restful positions that are more stable than a sphere. And it is great for positioning legs while you rest or sleep. You'll see these pulled out when a laboring person gets an epidural, but they're helpful for resting during early labor at home, too.
Like with the Birth Ball, you don't need to pay the "pink tax" and buy one called a Peanut for Labor. There are generic options designed for Pilates and other exercise that will be fine.
I like this one because, like the Birth Ball I recommend, it comes with a guide to help you use the tool to its fullest.
There are many sizes to peanuts, but they fall into two categories - horses and ponies. Horses are the ones, like this, that are bigger than your thigh and, when put between your legs, hold your pelvis in a simulated squat. Ponies are smaller and are great for propping up knees and separating ankles. For most labors, the horse is going to be more useful because pillows can do most things the ponies can do.