If I remember rightly, pink fiberglass insulation is used in construction and is measured in R value, insulation for smokers is different and is measured in K value, you might want to google it and then check in the yellow pages in your area for prices.
I built a portable "pit" that used this type of insulation and it was rigid, customer wanted 6" which was over kill, I think 1" was good for 1000°, back then it came in 2'x4' sheets.
Gene