The Classic and Vintages Sales section would be a good start. Otherwise there are auction sites on the web as well as craigslist where you might find some secondhand vintage rims. Otherwise your LBS could be a good source, ask them. Another way to get cheap rims is to buy a donor bike with 700c rims and use those.

Edit: Didn't see you are in Oz. Shipping could be a lot for rims, so I guess you might need to go local.
If there are any bike co-ops in Sydney they will probably have a few decent old rims kicking around if you ask.