For my son's first birthday we wanted to do the same thing and had trouble finding such a kit. We ended up buying a set of finger paints and used some butcher paper. I then picked him up and used him like a giant human stamp. We'd had a big piece of paper for a tablecloth at his birthday party to decorate, and after he got the hang of being "stamped", we made a couple of tries on smaller peices of paper to scrapbook. It looked best after the paints had been swirled together some. And we both had a lot of fun with it.

Micheals Craft store has the kits.

Crayola makes a great substance called Model Magic. It's like play-dough but very light, and dries flexible. I've used it over the years to do my kids handprints and it's much easier than plaster.

You can buy it at teacher supply stores. Sometime Kmart or Target has it, but in little packages for a lot more $$ than the teacher's store.