If it's PVC you're looking for, one way to get a large tube is to look for sewer drain pipe. 16" diameter is pretty big. Not likely to find that in the common hardware store.
Since it usually comes in 30' lengths for the city to bury under roads, what you're looking for are the cut-off ends that are left over from a construction project. Since it's new construction, don't worry, there hasn't been any sewage running through it yet. Look around development projects, new houses, commercial buildings, etc. I see this stuff lying around from time to time. Once, I even had a 24" piece in my backyard when my kid was small to play with.
But wait... what diameter of blades are you planning to make?
Do you really need to start with a 16" tube?