Antique malls and flea markets. Day trip out of town if you are into that sort of thing. It is more effort than browsing the internet or library and it might be a fruitless venture. There is a lot of junk out there and you can spend hours browsing. I've only been a few times but I also like finding old arithmetic books even if I was weak on "math" in school. Disappointingly, I haven't found any old history schoolbooks but I will keep an eye out. Although I have come across Encyclopedia sets, the seller is typically far too proud of them.