Maybe magazines or material that are specific to this subject matter. The library would be a good start. You could talk to the experts. You could order free material that may help in your quest. The internet is always a good source for information.
Mail order is the same way.
When I started in mail order I spent hours and hours of my time looking over material that I acquired at no cost to me. There was always someone who was willing to reveal a little piece of the pie. I began to piecemeal together what it took to become successful in mail order. From this I formulated a plan of action. Before you spend one red cent on mail order, establish direction.