I don't think Twain's _Buffalo Express_ writings have been collected.
Someone published, a few years ago, a pretty good bibliography of
Twain's articles (called "Mark Twain's Contributions to the 'Buffalo
Express'" I think, and published in American Literary Realism?). I
have the article at home, and I'll try to verify the source for you
if someone doesn't do it first.
I'm lucky enough to work right across the street from the Buffalo and
Erie County Public Library, which in addition to a nice Twain collection
(including the original manuscript of _Huck Finn_) has all of the old
Buffalo papers on microfilm. I've spent a good deal of time looking
through Twain's _Express_ articles. I can't imagine where else you
might be able to access these, since the paper would generate little
interest outside Buffalo.
Some of the articles are interesting, although in my opinion there's
nothing fantastic there. There are some minor gems, including a
piece called "Hanging to Slow Music" which deals with lynching. I
don't know if that's been reprinted. Many of the editorials are
unattributed, of course, although some seem clearly Twain's, and others
are likely the result of collaboration with others at the paper.
Twain's life in Buffalo wasn't particularly happy, and he didn't
produce his best work here.
The _Express_ stuff is interesting, though, and would be of interest
to serious Twain scholars-- but not, I think, to the general public.
If anyone is ever interested in traveling to Buffalo for _Express_ or
_Huck Finn_ research, drop me an E-mail. I'll be happy to hook you
up with the right people.