Knitting in the Old Way by Priscilla Gibson-Roberts and Deborah Robson includes a chapter on the Cowichan sweaters of Canada, which the authors call "the only true folk sweaters of North America..."
Besides the history of these sweaters, there are patterns for a cardigan and a pullover, plus information on knitting the distinctive details such as the shawl collar and unique shoulder join. There are also charts for some typical Cowichan motifs that can be used on other knitwear.
You can order Cowichan sweaters from companies that hire Salish and other tribal knitters, but these appear to be commercial enterprises. (Google "Cowichan sweaters" for a sampling.) While they may send you a certificate of "authenticity," I don't think there is a copyright issue here. A specific sweater pattern by a specific designer has automatic copyright protection, but not a whole design category, no matter how traditional. Can you imagine if someone claimed to own the copyright for cardigans?