The Electronic Text Center at the University of Virginia
<http://etext.virginia.edu/railton/> has the full texts of Innocents
Abroad, Those Extraordinary Twins, Tom Sawyer, Huckleberry Finn,
Connecticut Yankee, and Pudd'nhead Wilson online.  It also provides a
search engine to access all these texts at once.

A search for "sold down the river" turned up four hits, all of them in
the text of Pudd'nhead Wilson.  Perhaps there is no point stating the
page numbers here, since the e-text is that of the first edition of the
book.

It is a pity that the full corpus of Twain's writing is not available in
electronic form.  This would make searches such as this so easy.  Putting
Twain's writings online would be a worthy project for the Mark Twain
Forum to encourage.

Charles and Mary Boewe