This pattern calls for "1/2 " foam" ...what kind of 'foam' is she talking about? I'm thinking styro-foam, that can't be right...I would think some kind of 1/2" batting (or thicker) would be softer for baby? Foam...? Maybe memory foam???
I think she means foam used for making seat coushions. You can buy it by the yard at JoAnn's fabric. If for some reason you need to cut the foam I have found that using an elictric carving knife works well.