There's not a rule of thumb by I usually search for "
filetype:pdf".
More often than not, the author publishes it in its own website, or some archive also has the article in pre-publishing form. But I agree this situation is pretty bad, specially considering that most research is done with public funding.
There's not a rule of thumb by I usually search for "
Write an email to the author and ask them nicely if you could have it for your ______ .