Can you get your hands on one of these? Most of the groups use the Japanese fold Moleskine Pocket Sketchbook. The portrait groups don't require this book. You might try one of Moly's watercolor books, or larger regular notebooks, or even some other brand. I recommend moleskine. It is durable and consistent with most of the other artists' books, small, cool pocket for little surprises, gifts or collections. It is small so you won't have to spend a lot of energy to fill a page, 3.5"x5.5" roughly. Amazon.com, Moleskine.com, book supliers like Borders and Barnes and Noble carry them.
Start any time. Ready, Set, Go! I will recruit 3 more artists. Maybe you have a talented friend who would like to join us.