well if you are not worried about the tube once the fiberglass tube is finished then home depot or lowes (not sure where you live so not sure if you have those near you) have 8" concrete form tubes for cheap. Just bring a tape measure with you because those tubes can be 8" + a half inch so if you want multiple tubes then make sure they are the same diameter.
You may want to check out PVC pipe. If you can find a place that doesn't scratch up its inventory too badly, the exteriors are smooth, hard, and straight, and the pipe cross-section is usually very consistently circular. Not too expensive, either-
with a bigger tube you could wrap a concrete tube in Mylar/plastic and used that as your smooth surface. I am having trouble finding the how too but it is possible