How to install emacs-w3m via melpa?

[Richard Cobbe <rcobbe@rcobbe.net>]

I'm trying to install emacs-w3m for emacs 26.1, running on MacOS.
According to the emacs wiki page for w3m
(https://www.emacswiki.org/emacs/emacs-w3m), there's a convenience package
in MELPA, and looking for it on the MELPA website suggests that it does
indeed exist: http://melpa.org/#/w3m

But when I start emacs and run M-x package-list-packages, I don't see it in
the list of available packages.  I'm assuming that I've got something
misconfigured here, but I'm not familiar enough with the package system to
know where to start looking.

The relevant details from my .emacs file:

    (require 'package)
    (add-to-list 'package-archives
                 '("melpa-stable" . "http://stable.melpa.org/packages/"))
    (package-initialize)

And, if it's relevant, the value of package-selected-packages is

    (cargo rust-mode ledger-mode haskell-mode auctex)

What am I missing?

Thanks,

Richard