Being in Australia, I started out with exactly the same question, xiahouzi. I've found that in attending vege swaps (regular, moneyless, small community trading events), people are always swapping seedlings and seeds as well. This is a really useful way to source quality seeds, given they're simply backyard gardeners saving the seeds from their favourite veges.
I've obtained or given away seeds for beans, marigolds and mustard greens in this way already this season. And I've received or given away seedlings for Lebanese cucumbers, rosemary, Black Russian tomatoes, aloe vera, various lettuces, parsleys, basil and a foot-tall apricot sapling! That would give you an idea of how vigourous the swapping can be.
My local vege swap at West Croydon (which anyone is most welcome to attend if travelling through Adelaide) is one of Australia's best examples. See http://users.tpg.com.au/vegeswap
There is a list of regular swaps around Australia at http://www.ceres.org.au/node/114 but that information appears slightly outdated so you might have to do a bit of localised research or try to start your own local swap to proceed.
I'm not sure if the concept is widespread internationally, but if someone else knows, feel free to chip in and add to this answer.
UPDATE (24 Feb 2012): I've since discovered dedicated seed swaps are also occurring all over the country. If you can get in contact with one person from a local gardening or seed saving community you'll be pleasantly surprised at the passion for seed saving that occurs quietly without much reflection of that activity on the web.
Recently I went along to a local seed savers meeting and picked up a year's worth of vege seeds and some flower bulbs to attract the bees. Many of these were seeds you can't easily buy because they're the offspring of plants cultivated over many years by some dedicated gardener, shared in the gardening community and typically genetically distinct in some way that home gardeners prize.