I bought my supplies at Hobby Lobby. You can find rolls of tulle in the fabric section with the ribbon and in the wedding section. FYI, Dollar Tree has white tulle for $1! I used red, hot pink, & pale pink tulle for my Valentine tutu.
I also bought a package of non-roll elastic for the waist band.
Since I made a newborn size tutu, I really did not use much tulle. One spool would be plenty.
Here are a few tutorials that I consulted for info (the 2nd one is fabulous – showing a quick way to cut the tulle):