There are hundreds of late flowering clematis to choose form. Have a look at this site and enter September in the flowering period then click on Search and you'll get a long list http://www.clematis.hull.ac.uk/new-clemlistsearch.cfm You can also do a search on colour or height or aspect.
The info available includes the pruning group, flower description, height of growth, flower colour etc. Another part of the site explains pruning.
All clematis are hungry, thirsty plants. They need a good soil with plenty of well rotted manure and/or garden compost to enrich it and you can give them a handful or two of specialist clematis food every spring. They should bever be allowed t oget thirsty, especially in pots. I give occasional liquid feeds of rose or tomato food between spring and flowering and I dead head Group 2s to encourage a second flush of flowers later in the season.
Having said that, a few of mine have perfomred poorly this year whilst others have been spectacular. I put it down to stress from a long cold spring and then a hot dry summer so am nurturing them along and hoping for better things next year.
Another excellent site is the following