[TikiTDO]

The important and popular ones are absolutely available, but those are usually important because they have entered the realm of "common knowledge," at least in a particular sub-field. These are going to be at the top of the list when it comes to digitizing useful historic records. It's fairly easy to OCR a PDF, so as long as someone with some time decided "hey, this might be useful" then you'll probably be able to find it.

If you're doing databases then you've almost certainly been exposed to Codd's work, if not through his papers and books, then at least through textbooks and lectures. There are countless blogs, lecture series, and presentations that will happily direct you there.

The challenge is that there's also a mountain of work that never really got much popularity for whatever reason. Say a paper was ahead of it's time, or was released with bad timing, or simply kept the most interesting parts until the end where few people might have noticed. It's these sort of gems that are hard to find. It's hard to even know how many of these there are, because they are by definition not popular enough for most people to know about them.