Perhaps because the time between now and September for many parts of the northern hemisphere is the worst possible time for shipping plant material as well as the least desirable time for planting? Successful mail order operations understand this and elect to do the majority of their business during those times of the year when shipping and planting conditions are optimum.

Scootini..most stop shipping because it's too hot for the clematis and they don't want them to die and have to replace them...you can try some of these nurseries:
http://www.fairweathergardens.com/pages/vines.php
http://www.collectorsnursery.com/html/perennials_a-c.html
http://www.bussegardens.com/onlinecat.cfm
http://www.fieldstonegardens.com/index.cfm?page=secure
http://www.greergardens.com/clematis.htm
http://www.songsparrow.com/2006new/plantlist.cfm?type=Clematis&startrow=1&pageType=plantlist