Since the demise of Bernards, I'm pretty sure there's nothing in Melbourne CBD these days. Finding gaffed coins in Aus currency used to be super hard and super expensive, but there's some people on ebay these days who offer them for much better prices than used to exist. Can't vouch for quality on that front though.
The lack of bricks and mortar stores is really sad. You used to get a lot more from in person service at Bernards than is possible online.