And always check used bookstores. You can find gems there, and often take chances on new things because they're so cheap. And don't forget libraries! I could wax poetic about all those places.
In Germany we have booklooker.de as a market place for used books. I recently bought an autobiography from there which has out of print for many years. And I found the experience of holding a book in my hand older than myself strangely exciting. That book was printed ten years before I was born. The pages had taken on a yellow shade. Of course I held plenty of really old books in my hand by now. But only then I realized it might become a habit. Especially when there is a choice.
You want to read a Mark Twain? Why not by an edition from before the Great War?
I'm well aware of booklooker, and it's amazing. There's also bookfinder.com here in America. They're some of the best places to start looking if you want rare/old/niche books.