Buy a used copy of the main textbook and grammar notes from a second-hand bookstore. In Japan, Book Off sells them for ¥200–¥500 each (less than $5). Outside Japan, check AbeBooks or eBay – expect $15-20 per book.
3A Corporation explicitly states on their website: "We do not authorize the digital distribution of our textbooks. Please purchase from official bookstores or authorized e-book platforms." download pdf minna no nihongo 1