At most building supply places you can buy epoxy resins that will do this. They can be molded and formed. This material comes in two cans or tubes. You mix it together, then pour it or mold it, let it cure, then break it out of the mold, and presto, you've got your plastic thing. There are lots of different kinds of these resins in different properties, strengths and colours. If you cannot find it at the building supply store, try paint companies or plastic companies in your yellow pages.