I normally order my DVDs from Amazon, and I've totally forgotten that Australia is only getting Supernatural season 4 DVD NOW. It's released this Wednesday 6th January 2010. Here are the bonus goodies from 2 sources.
If you want a FREE Replica Chevy Impala, buy it from Ezy DVD for $69.97AUD.
If you want a bonus disc featuring Comic Con, as in 'Get up-close and personal with the stars, creator and writers/producers of Supernatural in this informative and entertaining panel filmed at 2008’s Comic Con convention in San Diego', buy it from JB HIFI for $59.98AUD.
I would LOVE to get the bonus Comic Con DVD instead of the Impala. I think it's harder to get the Comic Con footage. Although I have already got the season 4 DVD, I think I might get a second copy.
A tiny comment: Supernatural season 1-4 are also available on iTune (Australia).