Try the GW Australia website!
And yes one packet has a reasonable amount, since you'll only be using it to fill in gaps between glued parts.
As a guide i used less than half the packet of putty on my ISA Fleetbox assembly.
Although its always good to buy a second packet of putty,just in case.
PS! You should also buy a Pin Vice (available from GW too).
Its a hand held drill, so u can enlarge the hole for the base on the metal ships!!
Its essential for the larger Babylon 5 ships!!
Or else the bases have a tendency to snap,due to the size and weight of the larger ships, its happened to me twice.