This is the course I did. Btw: even if you do the course through IDP, they don't actually write the exam papers, the exams are written by British Council, thus the exam papers are no different.please can you share the materials you used in preparation am writing next month with IDP?
https://www.ieltsonline.com.au/ielts-master
As well as some Cambridge IELTS books for practice tests (you should be able to find these online). The 'master' course also has some practice tests but I wanted additional ones. I'm not claiming the course was great or better than other resources, I haven't looked too much at other resources to compare. It was holistic though, such that you can follow the course and not really need additional resources, this is why I liked it.
It is likely you can find free materials just as good online, but you will just need to spend more time combing through the internet to look for it. I suggest you take a look at the 'IELTS' sub forum on this website for more info/tips.