Hey Welcome to Craigieburn...!!
Best place to start is the big caravan shop in Sydney Road opposite the Ford factory. They sell heaps of stuff and I would bet they would have a water tank suitable for your TS. Alternatively, there are a few caravan factories in the industrial area in Campbelfield who may be able to help.
The opther place to try would be Red Rock campers.
http://www.redrockcampers.com.au/main/page_contact_us.html
Good luck...!
Darcy