There is a great coffee table book titled "Barn: The Art of a Working Building" by Endersly, Greenwood, and Larkin. It has many models of traditional barn styles, some of which have been converted to homes. It might be possible to derive plans from the models. At the very least, the book is full of design ideas.
You might also consider buying an old barn from a broker. I believe there a few listed in the resource guide and E-bay often has a couple listed for auction.
Good luck.