Thank you all for this lead.  I've found copies available on Amazon,
some for rather exorbitant amounts and others for less than $20.  I've
ordered a copy.  Meanwhile I did find a source of pdfs at California
Digital Newspaper Collection, cdnc.ucr.edu The pdfs are readable but the
scanned text is almost useless.  If you wish text copies you'd probably
save time by just typing the letters out rather than trying to correct
the scanned text.

My avatar has been doing readings from The Innocents Abroad in Second
Life, the virtual on-line world.  I created a web page from the first
chapter that includes a link to the YouTube recording as well as an
image of the Letter form Mark Twain published August 18, 1867, written
June 6, 1867, the day before the Quaker City left the dock.

http://bsh.bscottholmes.com/twain/innocents/chapter1