The cheapest place to purchase your cylinders is from www.kegking.com.au or one of Keg Kings distributors, but i'm a little bias because I am good mates with the guys there. But it's a pretty good deal. They are $190 and they come with a lifetime warranty and servicing. So you will never have to pay for hydro testing or replacement of valves and burst disks.
2.6kg cylinders are a good manageable size and fairly safe. The size of the cylinder should also be chosen to suit where you are using the system.
If i had a confined bar room with little ventilation i would definitely not use any more than a 3kg cylinder. In the case of a leak, a concentration of more than 30% co2 can be lethal. 3kg will be enough to make a 50cubic meter room a lethal environment.