While I don't know exactly what the Friends use, I do know that many siding patterns are available in yellow pine. If what you need is a standard pattern, you should be able to get it through a full service lumber yard. There are also several websites that show redwood, pine, and red cedar. The site below shows several patterns available. A lumber yard should have the patterns in a book, I would take a piece with me to see if it matches. There are also companies that will mill what you need, but it gets expensive.