How do you find critical analysis for a book
There are a few options! If you are looking online and you need a fairly basic analysis (a run down of themes or motifs, summaries, character profiles, etc.), there are a lot of websites made to help you -- including eNotes! You might want to check out the Literature Guide section and see if what you're looking for is there. We have a large number of great teacher-written analyses right here on the site.
If you need a more detailed analysis, or if you are trying to find an analysis that tackles a specific topic (e.g. "the use of color imagery in the work of T. S. Eliot"), you could try typing the topic into Google Scholar, which compiles journal-published articles and books. If you go to an institution (like a college) that gives you access to journals, you should not have any problem accessing them. If not, you can try getting guest access through a local academic library.
Which brings me to the third option: the library! Not many people use a real library any more, but it still has a huge number of resources, especially for older books. Use the online catalog or ask a librarian and you should be able to find plenty of books about the work you're researching.