I don't know much about cannon cameras/lens, but for Nikon (I assume Cannon and other brands will be similar), buying stuff off ebay from a store in Hong Kong or something is way cheaper than buying in Australia.
With a little caution, and some good research, buying online/overseas can be relatively safe.
For second hand lenses, some camera shops (good ones, not Harvey Norman etc) have a variety for sale.
Also, don't be afraid to spend a bit more money on a lens. A good quality lens will make a fair difference, and when taken care of, last a very long time.
Search google, there are thousands of reviews on different camera lenses. Lots of comparisons between slightly different ones as well.