I'm pretty certain that no answer key for C&S exists anywhere, in any form. It might be worth writing to the book's current publishers, Bolchazy & Carducci, to inquire further, however: http://www.bolchazy.com
There is a C&S Yahoo! Group whose coordinator has posted some answers (you'll need to join the group, of course, to see anything): http://groups.yahoo.com/group/Crosby/
Finally, I worked my way through the first 15 or so chapters 7 years ago and am giving it another go. I'd be happy to compare answers with you, if you like.