I finally found the third volume of the Rescue Bots Second Season DVD, at Kmart Sunnybank.
The fourth volume showed up at a number of retailers a couple months ago, but the third volume hasn't appeared anywhere yet (even JB Hifi, who has the other 8 volumes at my nearest store).
So if you are collecting the Rescue Bots DVDs, Kmart is where you can find the third volume, for just $9.

Speaking of JBHifi, I saw this Animated complete series boxset about a week ago for $57.