Coles has started stocking ducks. They’re not “cheap”, but not ridiculous either. I think Glenmore meats also stock them, and they tend to have good prices. However, I think the general answer to your question is “Chinatown”. Last time I was at Market City (above Paddy’s Market) there was a butcher next door to the IGA with some very reasonably priced ducks. I didn’t buy any though, so i can’t vouch for the quality.