Cereal boxes are a great source for cardboard! Especially when you're making something you want to last. Glue your pieces to a flattened cereal box, then laminate (or faux laminate). Cereal boxes are a great alternative to poster board.
*NOTE: Glue the picture to the BACK side of the box. (Not like pictured below, lol!). Like I did for the faux lamination tutorial.
I did this for my "Holding Hands Around the World" kids. See: