I have purchased clematis plants from Silver Star Vinery, Joy Creek Nursery and Brushwood Nursery and have been happy with all three vendors. Silver Star and Joy Creek generally have larger plants grown in 1 gallon pots (and their prices reflect this), while Brushwood's plants are slightly smaller and grown in 1 quart pots which are well filled with roots (and their prices are a bit less). If you are wanting to purchase quite a few at once, then Brushwood would give you the most bang for your buck, in my opinion, plus his shipping costs are quite low. If you just want to start with the largest sized plants, then Joy Creek or Silver Star would be a better choice. Just be aware that for all vendors, some plants will already be sold out now.
Kathy

Here is a link that might be useful: plant size at Brushwood

Here is an earlier thread on Brushwood

Here is a link that might be useful: feedback on Brushwood Nursery

Another thread from February regarding another place to buy clematis

Here is a link that might be useful: Experience with Klehm's Song Sparrow

An earlier thread from January

Here is a link that might be useful: good place to order clematis