Brushwood, Silver Star, Joy Creek are some of the most reputable who also have a large selection of clematis.

I ordered for the first time this year from Silver Star Vinery and Donahues and would definitely order from both of them again. Great plants from both places. Donahues was cheaper but also smaller plants but still nice size plants. Most of Silver Star were bigger plants but more pricey. Can't go wrong with either place. Judy

I would add Garden Crossings to this list. Clematis and other plants I got from them were superb. Both Clematis had larger rootballs than Silver Star's ones.
You can also buy Donahue's Clematis from Bluestone perennials.