I get all my tulle from here: Tulle Wholesale - Tulle Ribbon
They have tons of colors and the tulle is very soft. I always order at least 3 per tutu, 4 if it's longer. I just made a tutu dress for my almost 2 year olds that used 6 rolls. I like to really stuff the tulle on there though so that it's really full and fluffy.
Hope this helps!