Best you can do if fasttech doesn't have replacements is measure the one you have and head over to ibtanked, they sell custom cut tubes, but you need to know the dimensions ( height, dia, thickness etc)
Got a 6 dollar micrometer from home depot years ago just for this, anytime I get a new tank, first thing I do is measure it out for replacements.
Now, if yours is in pieces I would head over to FT and post in the comments for that tank asking someone for dimensions