Don,
I would suggest an architectural supply store that also does large format printing. There is one in our town. 130,000. We were able to buy the perfect weight vellum that was inkjet ready. They carried a lot of papers I doubt many of us would see as it is a different market. You should be able to get some pretty shazamy cad drawings & presentation pieces. For the presentation & rendering pieces this would be the thread for paper.
Sincerely,
Michael