because that is a brand not a type of potato. This happened as a marketing gimmick to sell potatoes grown in idaho state. This is because the state of idaho produces a very large portion of the potatoes used by the public.
yes they can be but other places like local feed/farm stores can be too or you can order them on line from places like http://www.territorialseed.com/. If at a farmers market it is best to ask the person selling them if they are good for that purpose or if they have any recommendations.