Barbara and Cindy:
You would be limited to whatever books it has by Twain
and about him, but you might try Amazon's new "Search
Inside the Text" function to locate any possible quotation
you are looking for. The search would likely take you
quite a bit of time to pour through all the iterations
of the word or phrase you search for. If no one else can
recall this quotation, this is at least a start....
Darren Chiang-Schultheiss
Fullerton College