I became aware of paperback originals last April, thanks to an article by Bill Crider. He points out that some well-known crime writers such as Lawrence Block and John D. MacDonald, got their start in paperback originals, and he appends "A Reader's Checklist" of less-known authors who compare favorably to them.
Since he was writing in 1971, Crider recommended searching used-book stores for original copies of these mystery novels. Today those copies would be available only from specialty dealers such as Kayo Books.
Checking my libraries (The Mechanics Institute and The San Francisco Public) I discovered new copies of these novels from the 1950s, 60s, and 70s. Stark House Press has been publishing re-issues of paperback originals for several years now. They offer books by every author on Crider's list and many more.