Hi Andy.
Have you tried Barbeques Galore and asked about Igloo coolers there? I know they stock Igloo coolers and when I was looking they had the 5 gallon (19L) round Igloo's for about $99 (no bulk buy necessary) and the round ones were dearer than conventional rectangular/square ones of larger capacity, however I haven't had a look for a while. If they haven't got any in I'm sure they would order one in for you. Also try Allgoods.
Is $100 a good price for a square/rectangular cooler? I could have sworn you can buy them much cheaper than that. BigW for example has a range of them for not a lot of $$ if I remember correctly, so does Bunnings. When your talking eskys, shape doesn't seem to be a real important factor (regardless of what John Palmer says in his book), especially if you batch sparge. Also don't stress too much about heat loss. Plenty of people are using the conventional rectangular eskys as tuns with great success. I'm sure other members here will throw up different brands of esky that they use which are perfectly suitable.
I bought a cuboid Coleman esky for my mashtun from ALLGOODS in Glenorchy, it's 19L and works great and cost me about $55 on special. They also had one that was about double the size. I bought it because I wanted a deep grain bed for fly sparging-if I had my time again I would have just bought a normal one and batch sparged.
Basically, although I haven't priced eskies in a while, that doesn't sound real cheap to me. I'd say you could save yourself some dollars by buying a normal shaped esky. I'd be keen to know what others here think but I hope it helps. Also, consider buying a secondhand one from a secondhand shop as they can be cheap as chips there.
Which homebrew shop are you referring to by the way? I find the guys at Brew by You in Glenorchy to be excellent blokes and they have the best prices on bags of grain you'll find in Tassie by a long shot. Good blokes. Hope it helps.
Cheers, Justin
Edit: added my esky brand and photo.
Few on ebay too: here's one:
http://cgi.ebay.com.au/Esky-Brand-Esky-50-...1QQcmdZViewItem