Even those pulps that are not available for free as public domain works have seen fairly extensive reprint and re-release efforts, so a wealth of pulp fiction is available right at your fingertips!
One of the more expansive resource sites on the Web for classic Pulp is ThePulp.Net. A more recent addition to the site is their fairly extensive list of digital pulp sources.
To quote the site: "Pulp magazines and reprints aren’t the only way to enjoy the works of classic pulp — or New Pulp — fictioneers. Many readers find e-readers, tablets and even mobile phones a convenient method of perusing pulp fiction. Our new digital pulp page adds new links and consolidates others from pages elsewhere on ThePulp.Net."
One of my own personal favorite sites for public domain ebooks is Feedbooks.com. I've downloaded dozens upon dozens of classic public domain fiction, both from the pulp and pre-pulp era. I have extensive ebook collections from such writers as Robert E. Howard, L. Frank Baum, Arthur Conan Doyle, H. Rider Haggard (the Allan Quatermain series), Alexandre Dumas, H.G. Wells, Edgar Allan Poe, Edgar Rice Burroughs, and many others, all very professionally formatted and completely free. The "professionally formatted" bit is especially attractive, as I've found that those free versions of public domain works commonly available on Amazon, Barnes and Noble, and others, suffer from exceptionally poor formatting, because these services would rather you bought their higher-priced pay versions.
Another nice thing about feedbooks is that you can, once you find a story you're looking for, click on the author's name and get a complete list of available ebooks by that author. Some authors have clickable sub-listings for series as well (Howard, for example, has a sub-listing for Conan stories). Alas, it's difficult to search the public domain stories--they don't come up in a normal search since Feedbooks is first and foremost, a retailer, but with a scant few minutes of searching the public domain sub-headers (Conan, for example, is under Public Domain, then Fantasy), you can usually find a story by the author you want, and it's worth it for the superior quality of the text.
Since as we all know, the best place to mine for RPG ideas is the deep well of previously-published fiction, the digital age is a blessing in making many of these classic public domain works available at the click of a button.