I rent them from Chegg
. You can also buy it from them too if you decided to keep them.
They cost about the same If I were to buy it from my current college compared to them. But I rent them instead. They would've cost $500 (including tax) to buy new from my school, and I end up paying only about $200 to rent them for the semester. So I saved about 60%.
You can try places like Amazon and Ebay too. There were another site I was recommended, but I forgot.