Where can I buy a sign in book for a baby shower?

Instead of a standard-style sign in book, consider a baby-themed scrapbook.  One (maybe 2 if it's a huge party) sheet(s) of baby-themed scrapbook paper can serve as the sign in pages for guests.  The remaining sheets throughout the book can be used for photos from the party. and for other memories that can be given to the mom-to-be.  These craft items can be purchased at any craft store, such as Hobby Lobby, Joann Fabrics, Michaels, etc.  The larger the craft store, the bigger the selection.

http://www.signinbooks.com

Amazon has a CR Gibson Baby Shower Keepsake and guest book selling for $15.78
specifics:
Baby Sakes has a Ribbon bound guest book selling for $45.95
specifics:

This ribbon bound guest book is perfect for recording guests' well wishes at the baby shower or other events such as christening. The ribbon bow ties around the entire book yet the book remains able to be opened - the ribbon even serves as a page holder.   This guest books are handmade by the Rag and Bone bindery and are true boutique quality. These Guest Books lie flat when opened and contain 100% acid free pages.   Choose the printed version with 360 signature lines or the blank page version (twenty pages). The overall size is 7" x 10.5" and the page size is 6.5" x 10."   About the Polka Baby design: The Polka Baby cover option is a printed design on creme linen paper and coordinates with the Rag and Bone Bindery Cherry Blossom baby book, brag book, photo album and scrapbook.

	Features Overall Size: 7" X 10.5" Page Size: 6.5" X 10" Pages Printed (360 signatures) or Blank Page Count: Twenty 100% Acid Free Pages