Dollar Tree is great for those kind of trinkets. As is Oriental Trading Co. online.
Since you mentioned books, I've ordered directly from Scholastic before. Look for their specials. I got book #1 of the Guardians of Ga'Hoole as favors for my daughter's birthday party for $1 each.
Another idea would be to give the kids un-decorated sugar cookies. Make up a price list of things they can buy to decorate their cookies- icing $,40, candy corn $.02/ea, etc. Give them a blank "order form", a $1 spending limit and require them to total it before they collect their cookie decorations. A parent can be in charge of passing out the decorations and checking the math.