Louise Cooper - Time Master trilogy and the other three related trilogies, the Indigo series and various stand alone books. I've actually gone to her website, and reached out to the agent that is listed there. I didn't get a reply; I have no idea if they still represent her estate after her passing.
David Eddings - yep, I know I could 'travel' the UK and get them. Maybe if I didn't have paper copies I'd get serious about doing so.
Robert Aspirin - Myth Adventures series; the second one was released in November, so I've hopes that I'll be able to get all of these soon.
Hal Clement - his most popular stuff is available but that's it
H. Beam Piper - Fuzzies and Other People