you can get plaster of paris in most hardware stores (e.g. home depot, lowes) or in building supplies stores.

You get plasticine cheap in a dollar store. If you want water based clay (for larger projects) you can check art supply stores or ceramic stores (check yellow pages and phone them if they have it before you go there). It doesn’t really matter what kind of clay you use.
You will need “fossils” e.g snail shells, mussel shells, plant leaves. Press your “fossils” into the clay so that they leave an impression. Brush the clay with cooking oil or similar. Mix plaster and spoon on the impressions you made until they are filled. Let the plaster harden.