I was given a dead dog bag by a vet friend, but she said they cost $25! (Maybe she wanted to make sure I didn't ask again). Very strong and durable. I cannot find a retail or wholesale outlet for them. Regular garbage bags tear too easily, I use the "weed" bags from the gardening section of the supermarket, 3 for $4, but they will eventually tear if you treat them too roughly.
Kathmandu lists this:.
http://www.kathmandu.com.au/pack-liner-kathmandu.html, $14 probably fine but, as usual with Kat-man-don't somewhat overpriced, even though on sale. From time to time I see the equivalent in Paddy's Mountain Designs etc., but they never seem to be a regular stocked tem.
Pack covers are one of those products designed by Marketing, completely unsuited for purpose, but sell on looks. Another option for uni students is just wrap everything twice in supermarket shopping bags - they will work pretty well for all but full immersion, weigh close to zero, and it's good to re-use plastics.
Skibug