I think we are talking about what are commonly known as Piper`s hose. The double fold gives a thicker looking cuff, and perhaps a bit more adjustability on various leg lengths. They are indeed very popular with pipe bands, they are widely available in various colors, mostly synthetic material, and go for 20.00 to 30.00 bucks a pair. I have a pair, and I think they are a good value.

http://www.jhiggins.net/footwear/?sort=featured&page=2