I live in the cool Pacific NW so I don't have first hand experience. After reading posts from other gardeners it seems the type 3 pruning group does best in the hot south. The late large flowering group, the viticellas, and the texensis hybrids seem to take the heat the best. There are many mail order sites where you can order clematis. Joy Creek, Brushwood, Silver star, Donahues are a few that carry clematis. You can go to clematis on the web and go to their search engine. You can put in pruning group 3, the colors you desire and it will show you photo's of clematis to give you some ideas. Jackmanii is a type 3 pruning group with large purple flowers. It has been around for 150 years and is so common. Every nursery, hardware store,and big box stores all carry it up here. I am surprised clematis aren't stocked down south, you may have to mail order everything. Prince Charles is also a nice blue viticella type that blooms for a long time up here.

Here is a link that might be useful: Clematis on the web