Buy secondhand. Sources include stores, flea markets, and web sites.
It can be difficult to find some things…
If it’s a sefer, I heard about a place in Yerushalayim where they’ll print and staple together for you any sefer in the Otzar HaChochmah (the library hard drive), and I’d assume there’s such a place in Boro Park too.
You can try public libraries for books. This probably works best if you live in New York, or a similarly large city, as interlibrary loans tend to take very, very long.