The best literature ever written is often available almost free. All those books that HighSchool and college students are supposed to read are available at your nearest used bookstore or garage sale in paperback (often unread) for pennies. There is a reason why students are assigned these books to read: The writing is of the highest quality, and the subject matter stands the test of time. Go stock up.
Or on the Internet:
and your local library.
Here are just a few of the books (in no particular order) which I have picked up for stray pocket change:
In the U.S., at least, the best place to find paperbacks really cheap are at resale shops run - Goodwill, Salvation Army, local religious organizations, where often everything in soft cover is $0.25. Of course, the cheese mentioned above predominates, but you can find amazing bargains.
Many libraries (at least in the U.S.) have book sales to earn money. Some of the books are retired library books, but most are donated by the local citizenry. There's an excellent web page with schedules for such book sales across the country: http://www.book-sales-in-america.com -- TimVoght
Often, people will donate books to libraries that are worthwhile, but slightly worn (especially paperbacks), and not added to the collection. They tend to go directly to book sales, generally for mere pocket change. -- SckotVokes
BookCrossing is a program that leaves books in public places for anyone to find, read, and pass on. Anyone can join.
You know, this assumes that the "best" literature was that written a hundred years ago, an assumption that's quite risible on its face.
I think you're the one who's introducing that assumption. Look at the list of books given by TimVoght above. Out of 11 books, two were written in the 19th century; the rest were written in the 20th century. including five that were written in the last 50 years.
The question is why people think that the best literature is old literature. To that question there is a simple answer: there are too many authors nowadays for any one of them to capture the imagination of the entire world. This doesn't mean that literature in the past was any better either on average or at its peak, it means in fact that literature in the past was extremely sparse.
No; it means that we've had time to forget about all the crap that was written in the past, while the crap that's being written today is still fresh in our (collective) memory. Ever wonder why a much greater proportion of old architecture than new architecture looks good? It's because the old buildings that looked good were kept, and the ones that didn't weren't.
See also: BookShelved, GreatBooksList, LiteratureBookList