For my original self-published books, I’ve been using three primary publishing routes: Amazon, PayHip, and Draft2Digital. Draft2Digital distributes to Barnes & Noble, Kobo, iBooks*, and several other smaller retailers and subscription services. They also keep adding new distribution channels, and so far, I’ve added my books to those as they’ve come available.
What that means is that my four original self-published novellas and the bundle are available a bunch of different places beyond the big names: Inktera, 24Symbols, Scribd, Tolino, and Playster. You can also get them through Kobo’s subscription service, Kobo Plus.
(*A note about iBooks: the publishing requirements for iBooks are more strict than those for other retailers, which is why only the Hands On bundle is available through that channel. That might change at some point.)
As far as the second-edition stories I’ve self-pubbed, so far, I’ve only put them up on Amazon (and into Kindle Unlimited). I will have a collection of previously published short stories coming later this year, and that one will likely go into other sales channels as well.
At any rate, if you can’t find my books on your favorite retailer’s website, drop me a line and let me know, and I’ll look into it!