As far as finding ebooks, isohunt is your best bet. Start by searching using author's last name, and if you get any hits, also throw in the title and see which editions you can find.
There are also legit ebook sellors such as CourseSmart
but the only drawback is that you're paying 50% of the price of textbooks and yet you can't sell them again. So watch out.
As for reading off the iPad, it really depends on your reading habits in the past. Do you rely on textbooks to supplement lecture content? If you've actually read your past textbooks, then you will most likely put in the effort and read from your iPad too (and bonus of not having to carry them to Mac everyday). Even though I don't have a tablet, I've used ebook versions over textbooks (even if I have them) since it's just easier to open a PDF file and hit search and find a keyword to aid in my understanding.