Considering that every player is available for the right return, the number of such pitchers is quite large. The Mets have several superior to Miller. Miller might be more "available" than them, but that simply reflects the mentality of our FO in terms of what kind of return they are willing to accept.
But it remains to be seen if the Mets are actually willing to move their staters. And they really only have Harvey and DeGrom that are superior to Miller.