I used Estes Pro Series 2" tubing. I had bought some once to see how it compared to Semroc's LT-200 tube.
I really like the LT tube and was concerned it might become discontinued someday, so I was looking for a substitute.
Turns out the Estes tube is 2" O.D. while the Semroc tube is 2" I.D. Wall thickness seemed comparable.
After moving it around my bench for several years, I finally decided to use it so I built the Javelin.
Both my AeroDarts are LT-200, as is the Centurion I'm currently building.