The Brazilian but left is a liposuction and a fat transfer. The fat transfer is a non-vascularized tissue graft. When you put the fat in the buttocks it will live by blood supply. The fat that gets a blood supply will live in the fat that does not get a blood supply will die. When you sit and put pressure on the fat before it has a blood supply it will die. The fat is weaker in the beginning and gets stronger as the blood vessels grow. So the longer you go without setting the stronger the fat will be in the less fat you will lose. The question is how much time does it take for the fat to be strong. This is controversial and each doctor will tell you what they think but no one really knows for sure.
I have never told my patients that they can not sit for two months, but you shoudl follow your doctor's adivce. I do not usually recommend specific pillows either.
You can try a donut pillow, too. Most of the pressure will be on your thighs but it won't be as firm as the one you have now most likely.