The comments on their FB page about this very issue was enough to convince me not to get one. Looks like you may be pushed as none of the vendors appeared to carry spare tanks at the initial time of sale.
I don't ever recommend trying Fasttech to anyone, and I've no idea if they still do, but they used to carry a range of glass tanks for their popular clones. Might be worth trying that one? Or even seeing if there's a clone with similar tank dimensions? If THW aren't going to supply spares then clone alternatives seem to be the only solution.
A more expensive solution may be at Phiniac Rugged Pyrex Glass Tanks - they might custom make one for you if you ask them nicely.