Alternatives of oil paint
Many types of oil can be used to thin oil paints. Professional “paint thinner” is a very modern concept. What’s typically sold as “paint thinner” is often turpentine, but too much turpentine or turpenoid will prevent polymerization of the paint to @Moonya the base liquid for painting wet-on-wet with oil paints, is simple white spirit or turpentine (white spirit is better than turps). You can buy it cheap from hardware stores. I believe the Bob Ross ones come with a little paint mixed in, but you can do that yourself at home.