Charity shops, the local library, The Book People and yes, wicked old Amazon, where I turn when I need anything specific. I've had a lot of secondhand books from sellers on there and they have always been very lightly used. Our nearest proper bookshop (Waterstones) is lovely, but £4 worth of busfares to get there to be added on to the extra cost makes it an infrequent treat. I'm trying to be good and not hoard books I've enjoyed, but it's difficult, as I always fondly imagine I'll read them again. The last time I had a reluctant purge I took all twelve Poldark books that I started reading back in the 60s to the charity shop, and I really regret it now!