Well, I was going to mention JSTOR, but other people beat me to it. So I'll just mention
arXiv and
NTRS (although NTRS is a pale shadow of what it once was...)
Granted, arXiv has a lot of crank papers on it, but it also has many, many brilliant ones, and in some fields, such as particle physics (my field, as it happens) it's probably the best possible source to look for any given academic paper that's been published since its creation.
Far better than any (printed) journal, because arXiv has (nearly) all of them, instead of just whatever happened to have been printed in that journal, and of course is easily searchable. Call
this paper, for instance, disreputable because it's
on the Internet. I dare you!
NTRS, of course, is (was) a digitized repository for a vast amount of technical NASA material, including a great deal of mission planning work since the 1960s. I don't know in what universe (scanned versions of) official NASA documents or contractor studies discussing lunar bases or Mars missions would be considered "non-credible" sources for, at the very least, discussing NASA lunar base or Mars mission planning.
In any event, to return to the OP, my research for
Eyes Turned Skyward has been very extensive. For the most part, I have found, downloaded, and read a large volume of (mostly) NASA documentation related to various aspects of the timeline, together with some secondary sources (for example, Asif Siddiqi's books on the Soviet space program up to the mid-1970s, which are probably the premier academic source on the program, or Paolo Ulivi's books on planetary exploration, which are also highly regarded). In general these have been useful for establishing the base facts of a post, which we then "fill out," so to speak, with our own creativity. Both I and e of pi have made very extensive use of basic calculational tools to assess the feasibility of various things in our timeline, such as the performance of rockets (calculated using a generally well-regarded if limited on-line performance calculator) or the masses of lunar spacecraft. We have also used Wikipedia in some instances where either we were looking up minor or basic facts (such as the rocket equation, the date of the Three Mile Island accident, or the mass of Huygens) or where there were or are a comparative dearth of primary or secondary sources. As time moves on we must, of course, necessarily rely less on research as our timeline diverges more and more from OTL, especially in technical details and history.