There are a few ways you can get books not available online. The first is your public library. While your public library may not have the items that you need on hand, many public libraries are a part of a larger consortium, which means you can request items to borrow from other libraries nearby.
For more academic books, you can do a search in WorldCat Discovery. You can enter your zip code to find what institutions close to you have this item. For non-public libraries, you will need to contact them for their hours and walk-in policy.