I never miss checking out charity shops as they can be great value.
If it's something specific I'll look on ebay.
When I got into the old Ed McBain books I searched ebay and Gumtree and found an ad weeks old on Gumtree where someone had the complete collection of his 87th Precinct novels, (50 books) which are what I wanted. Cost me £30 with postage
But unusual to strike that lucky.