Kmart does one pretty cheap. Probably not the best insulation but a yoga mat will fix that...Could it also be worth picking up a cheap esky to use a HLT for sparge water (boil some water in the urn, then transfer to HLT before starting the mash process) ? Seen a few similar to this for $20/$30 on facebook - https://www.bunnings.com.au/esky-15l-blue-keg_p3240514
How have you been finding this system? I have half a foot in all grain and have been debating between saving for a Guten system, or just getting a boiler and modifying an old esky for a mash tun. Though the jacket, false bottom and BIAB look tempting to reduce the vessel numbers and work load.