Brohan said..
Just go to SurfHQ down in Sandringham, they should have a few for sale, or TheZu in St Kilda.
Both of those will sell good wetsuits and they always have something on sale to save you a bit more money.
If you want cheap, any of the sports or outdoor chain stores will have wetsuits perfectly adequate for Melbourne in summer.
Try Anaconda or Rebel Sports or BCF or Amart.
Aldi sometimes sell steamers for $60.
You can buy from Wetsuit Warehouse online, and their shop is in Cheltenham, just a short walk from the station.