You can also do the FAA ground school online (can't remember the link, but you should be able to search for it). Once you have successfully completed the online FTO's 'exams' they issue you with an endorsement to take the Lazergrade exams. You can then either take the exams here at somewhere like American Flight Training or wait until you go to the US and take them there. In the US they cost around $75 I think....
.....I've just re-read your post. If you have an Aus CPL / IR there is no requirement to do any ground school to get a FAA PPL/IR. You have to pass an FAA 'Instrument Foreign Pilot' multi choice computer test to get IR priviliges, but all you really need to pass that is to get a copy of the Jeppesen Instrument Commercial manual (in my opinion). Then when you go to the US to get your licence at a FSDO you can arrange to take the IFP test then.
Cheers
EA