Interlibrary Loan aside, if you live anywhere near a university with an academic library, you can usually buy a membership even if you aren't otherwise affiliated with the university. Locally, I know both UNC and Duke offer programs of that nature. I have previously kept up a UNC library card so I can use their math/CS library. Which reminds me, I should probably go renew that... I haven't been over there in a while.
Also, used bookstores, and Internet retailers like Amazon, Ebay, etc. Not all books that are "obscure" are necessarily expensive in the "rare, hard to find, valuable" sense (although to be fair, some obviously are).
Also, don't forget that community colleges also have libraries, and they're probably a lot more lax than large universities. For instance, one local community college even partnered with the city to create an integrated community-college library.
Also, used bookstores, and Internet retailers like Amazon, Ebay, etc. Not all books that are "obscure" are necessarily expensive in the "rare, hard to find, valuable" sense (although to be fair, some obviously are).