Beach Road Cycles opposite the old Brighton Baths has a range (!) of tandems. When we bought ours, they had quite few form $799 up to $4500 for a lovely Cannondale.
Before I ended up there, I started at local bike shops (MBC/BSC/Ivanhoe/StKilda) none of them were terribly interested and some of them pointed me to Beach Road. When finally I got there, it was a relief to find someone who would sell me a bike!