Abe books was my go-to in college for international editions of engineering textbooks. $40 for a book that cost $450 for the US edition. Only downside was that all the units were in metric and weren't perfectly converted, so I had to check against a classmate if we had work out of the book. All the info was still the same tho, and it was 10% of the cost, and it let me take open book tests where a digital copy wouldn't cut it.