The market for decorative stencils is enormous. You can purchase them online, in stores, or from catalogs. Stencils can be custom made to say anything you want. They are usually made with lasers and are very professional looking. Most of those are made of Mylar, since it's durable and flexible.
Most companies have a big line of already-made stencils for you to choose from, which is always cheaper than having one custom made. If you shop around to a lot of different companies you are bound to find something you like for your decorating project.
You can purchase a complete decorative stencil kit from some companies. These will come with everything you need. Included are the stencil, paints and the brushes. Of course you can purchase only the stencil with paint and no brushes, or just the decorative stencil alone.
There are many kinds of decorative stencils. You can get floor stencils and make a beautiful design on your floor, or wall stencils which can be used on the walls or ceiling of a house. Some companies even offer decorative mural stencils.
When it comes to paint for the stencil, the sky's the limit there also. There are indoor paints which can be used on most anything inside, including metal, ceramic, walls, wood, leather or canvas. You would purchase a different type of paint to use on decorative stencils that will be outdoors. These paints will work on stucco, concrete, stone, masonry and cast aluminum. There is also a special paint for metals, which requires no prepping. So choosing the right paint for your job is very important. Also, keep in mind that choosing colors from a computer can be a little tricky. Different computer monitors will show colors a little differently, so you may want to go to a store to choose your paint.
Places like Target or Meijer, and craft or paint stores, usually have pre-made stencils. You probably won't get the selection that you would get online, but if you visit a brick-and-mortar store you will be able to get your stencils and any other supplies you need right then instead of having to wait for the materials to be delivered to you. The same goes for catalogs: you will have a much better selection to choose from, but you will have to wait for the items to be shipped to you.