Kmoo - my top recommendation is these guys..
http://www.hydroponic.com.au/ Hydroponic Connection on Dorset Rd in Bayswater stocks Canna.. Go there on Sundays for a good chat with the salesperson, he's a funny dude.
Chronic Hydroponics at 31 Anderson St Templestowe stock canna too I believe, give him a call to check. He's a helpful guy, seemed honest enough though not the cheapest.
Green Lite Hydroponics at 271 Maroondah Hwy in Ringwood also should stock it. Again give them a buzz. They're the grumpiest store of them all.