Janice, being in Maryland, the closest supplier that I view as reputable and stands behind their plants would be Brushwood Nursery in Pennsylvania. Dan grows great clematis that can be directly planted in the ground without growing out in a pot. He is always willing to answer any questions you may have. The link to his nursery is shown below. There are many other suppliers of clematis that are similarly reputable but I am just listing the one that is closest to you and that I have dealt with many times. Chalk Hill, Joy Creek, Collectors Nursery, Seneca Hill Perennials, and many other nurseries out there sell clematis and are equally reputable but Dan's is the closest to your home and shipping is free if you order two or more clematis.

For me in my garden, the clematis that blooms the longest hands down is Petit Faucon. It begins blooming in late April/early May and doesn't stop until September in most years. Dan doesn't carry this specific clematis but Chalk Hill does. The one thing about this clematis is that it is not self clinging so you will either have to give it some guidance and support in getting it to stand upright or allow it to spill over the ground or even some low growing evergreens. Others that bloom a long time for me that do cling include Polish Spirit, Julia Correvon, Etoile Violette, and others in the viticella family as well as the Duchess of Albany. Several good things about the viticellas is that they are not as succeptible to clematis wilt, are very hardy and vigorous, and in my garden, will bloom a second time after the main flush of flowers if you cut them back by half and feed and water them well.

Good luck with your clematis!

Here is a link that might be useful: Brushwood Nursery

I would add Huldine to your list Miguel. One of the top non-clinging clematis which I also recommend to many is Arabella. People adore it!

Huldine
{{gwi:573176}}

Arabella
{{gwi:573177}}

Make sure you read up on clematis in the FAQ section added below:

Here is a link that might be useful: Clematis information