Somewhere in this thread will be a list of GW distributors, I got 4 bottles of S6000 directly from the SA distributor at $60 per bottle over a year ago. As far as S6000 goes I'm not sure if there will be any left as it was discontinued, and as far as SynX 6000 goes I haven't seen it yet although I have seen SynX 3000 in Repco, most likely you will have to go straight to the distributor for it (should be cheaper too).
edit: I couldn't find the list, but I just remembered at the time I emailed GW for the closest distributor via: http://www.gulfwestern.com.au/Contact%20Us.html