lets say I wanted to buy a book about a certain kind of fish where would I go?

I wouldn't " go" anywhere. I'd google.

I will add "Check what your library  has got" to Speed's answer.  The name the certain kind of fish might appear highlighted by "Google Books", but Google Books might not have all books that contain a paragraph about that fish.

I'd start with an internet search to identify books about that kind of fish.

Armed with titles and authors, I'd then seek a point of purchase. I like local booksellers and would probably ask them to get it for me. Amazon doesn't need all my money.

Barnes and Noble or Amazon.