What are you planning to house in it ?
Pretty hard to find, I know I looked a few years ago (nothing 2nd hand on GumTree at the time, not even double bay enclosures) so I built my own using plywood.
If you don't like working much with timber , check out the CONNECTIT system
https://www.metalmate.com.au/connect-it , using these joiners
https://www.metalmate.com.au/connect-it/connect-it-components/connect-it-corner-joiners (they are very strong even though made from plastic, can be found at Bunnings), you can build a nice multibay system using this and a few sheets of plywood (Bunnings will even cut to sheets to order, just call the local Bunnings and ask to be connected to their TRADE DESK).
Or simply buy some long threadbar and suitable sized nuts & washers , and some sheets of Project Ply (all available from Bunnings). Drill 4 holes in the corners of 3 sheets of Project Ply (120cm x 60cm) if the tanks you have are smaller than 120cm L x 60cm W) and adjust the nuts and washers supporting the sheets (shelves) of Project Ply so you have enough vertical clearance for the bottom & middle tanks, tighten the upper nuts onto the upper washers and you will essentially be done. Slip the tanks onto the shelves.
If you don't like the industrial appearance of bar bolts and threadbars , simply slip some painted PVC tubing or square hollow aluminium tube over the exposed threadbars and bolts.
Beauty of this approach is it's adjustable and easily disassembled to be moved elsewhere.