there are quite a few books out there, but most use the same HSC exams, so once you buy one you've bought them all.
i recommend the MANSW one, it has good clear solutions (sometimes with multiple methods)
my personal opinion is that if you need to do more than 4 or 5 past papers, then you should reassess the way you are learning; you are probably focussing too much on memorising the solutions to questions.
to prepare yourself for uni, most subjects will only provide 1 or 2 past papers if you're lucky (often without solutions) so you need to get used to learning the content rather than memorising exam questions. just my 2c