For example, check out Shapelock, which is a type of plastic, one that is easily moldable. You can use it for any sort of do it yourself projects around the house. It is a sort of polymer that's softening point is an extremely low temperature, so you only need to heat it a bit to make it moldable. Search online Shapelock, and you'll find that simply putting this product into an oven will allow you to easily mold it, and then it will harden as it cools down.
Additionally, there are tons of companies online that offer custom injection molding and other sorts of plastic molding. You can find out about many companies simply by searching online, or by checking your local yellow pages. Look for 'injection molding' or 'custom plastic companies' to find businesses that may fulfill your needs.
In the world of plastic molding, you need to find a quality supplier and product for your needs, whatever they may be. Look far and wide, and search on the Internet. Whatever you need molded, it can be done.