I have some additional native plants growing in my rock garden. Most of these don't require a rocky environment, but they often grow in rocky areas in the wild.

Arctostaphylos uva-ursi
Aster linariifolius
Hypoxis hirsuta
Aquilegia canadensis
Asclepias verticillata
Campanula rotundifolia
Silene caroliniana
Antennaria neglecta
Corydalis sempervirens (an annual for me, but self-sows)
Phlox subulata
Penstemon hirsutus var. pygmaeus
Potentilla tridentata

Leiophyllum buxifolium is a wonderful plant, but particular in its requirements. In the wild, it grows in areas of moist, acidic sand, and not, as far as I know, in rocky areas. I doubt it would object to growing in a rock garden, but be sure to give it very acidic soil.

Since you're interested in rock gardening, you might think about joining the North American Rock Garden Society. They have a seed exchange that is a good source of unusual native seeds -- I got a number of the plants listed above through their seed exchange or from other members of the society.

Here is a link that might be useful: North American Rock Garden Society

For Sedum nevii try the place linked. I've had good success with them. Also consider their Solidago roanensis, native to SEUS.

Here is a link that might be useful: Sunlight Gardens