Outsider art is not an institutionalized form of art and therefore cannot be bought at routine exhibitions. Since the artist works in isolation and on self-generated ideas, exhibitions of this type of art does not occur very often, especially in the life of the artist.
Nowadays the art is available at exhibitions, which are generally organized by an art collector. The exhibitions are often organized along with conventional, folk and naive art. You should look for advertisements of such exhibition in the newspapers and thus help yourself to buy some good pieces of outsider art.
In this age of internet, even outsider art is not away from the use of the technology as a platform for commercial marketing. Many collectors offer pieces of outsider art on sale, every now and then. You can search for the websites that specialize in sale and purchase of the outsider art and select the pieces you want to buy.
Wide networks of media outlets has made working in isolation for artists almost impossible. They now can be known in their lives while they continue to paint. This makes possible to buy the art even directly from the artists. Also, because of the awareness about the commercial need for promotion of the art, sometimes artists themselves appear on the surface selling their work. So you can buy directly from them.
You can also use social networking websites to place a message expressing need for buying outsider art, and if people have collection of this art, they can contact you. They can also help you contact people who might be having a collection of this art and want to sell it. You might not expect immediate positive results in this effort but it can be very fruitful sometimes. Also, better join the outsider art lovers group on the social network, you might be able to come in contact with a person who is willing to sell pieces of outsiders art.