You might find most of the information you're looking for at this site. As far as mail order goes there is a recent post on this forum listing the favorite sources of people on the forum.

If you read the threads on this board you'll find loads of information.

The pruning types are based on how the Clematis bloom. Some bloom on new growth only, some bloom on old and some bloom on both.

Hard pruning type IIIs also keeps the growth and blooms lower on the plant. If you never prune them down you end up with bare vines with all growth and blooms at the top.

On Type IIs if you never prune you often end up with long scraggly vines with few flowers or a big tangled ball of dead stems.

Two good sites:

http://www.clematisinternational.com/

http://www.clematis.com.pl/wms/wmsg.php/