I used my first bone folder in a book binding and box making class in college. They are made for creasing, smoothing, and scoring paper, but can also be used for sewing, pottery, and curling paper ribbon.
Bone folders are traditionally made from the leg bones of deer, cows, elk, or similar animals. Ivory, wood, metal, bamboo, and Teflon are also used. I've only used a plastic and bone version. They also come in different shapes and lengths.
If you do a google search for "bone folder" you will get folders made of ALL KINDS of materials. My suggestion is to specifically search for your desired material. For example, if you want a teflon version, search "teflon bone folder." Many companies call their folders "bone folders" when they aren't made of bone so be as specific as possible in your search. If you want one made of bone, searching "traditional bone folder" or "genuine bold folder" can help to weed out non-bone versions.
Prices vary based on the material and size. I've seen $6-$45 for one folder. It's worth investing in good quality folders. As for size, experiment to find what works for you. I prefer longer because I have bigger hands. You want to make sure it's long enough to get a good grip.
Here is a set of 4 where each folder is a different shape. This Teflon set also has different shapes.
Here is a 35 minute video that compares bamboo, teflon, plastic, and bone.
If you use a bone folder, I'd love to know what size, shape, and material you use and what you think of it.