I know I keep on about public libraries (sorry to be boring!) but lots of people are unaware that libraries often sell off books they no longer need, or which are slightly damaged, or which have been superseded by a later edition.
I recently bought a huge Collins English dictionary for a tenth of the original price, and often get novels for 20p or 50p and hardbacks for 50p or £1.
Many libraries have a permanent shelf or box with books for sale, and some have one-off sales. And you don't usually have to be a library member - just pop in and rummage!!