How did you do your BAK, you might be able to save me some coin here mate.
ABX, mine was a long time ago but have gone all the way from BAK through PPL, CPL, IREX and some of my ATPL subjects on self-study alone.
As others have indicated, a good text book and plenty of practice questions. I think I was fortunate in having a maths based study background so understand that others may choose to do either a course or some 1 on 1 study for later parts of your flying exams but the BAK is a very simple exam that requires primarily memory work only - a course is not going to teach you how to remember stuff.
The other key, must, compulsory, mandatory, essential, required (am I making myself clear?) item you need is the CASA sylabus - downloadable from CASA for free. If it's not on the sylabus then they can't ask you about it - read the document and make sure you understand each of the terms/areas the list and you can't go wrong.
Hope this helps you save those $$$ - no school or other organisation has the right to force you into their study course as a part of your training program - any school that does is not worth training with.
UTR.