After half an hour searching around the Internet, it seems like maybe all of Charles Portis's works are now available only from your local library, or as eBooks, or as (strikingly expensive) used books.
Actually, there may still be a paperback run of True Grit out there. But Gringos, Norwood, The Dog of the South, Masters of Atlantis, Escape Velocity; they all seem to be entirely out of print.
At least, if I'm reading the tea leaves accurately (how does one search for that question, exactly?)
I wonder how common this is, nowadays; are publishers finally ceasing print runs of authors even this popular?