Here are several listed in Dr. Waldman's article:

'Claydian Variegated' A tissue culture sport of 'Madame Masson' from Clay Nurseries in British Columbia with leaves having a distinct white border enclosing a green center. White flowers touched by a golden flare bloom in May. It is a well shaped plant and hardy to -15^F, but is a slow grower.

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'Goldflimmer' A German introduction with splashes of sunny yellow in the center of the leaf. It is probably a R. ponticum sport with rather non-descript lavender flowers in May. It is an excellent grower, rather vigorous, and propogates easily. The plant is well shaped, and it is hardy to -15^F. There is a slight tendency for reversions of all green shoots which should be removed. Similar plants include 'Sunsplash' which was found in Malcolm Whipple's garden on Long Island, NY and a large unknown plant growing at Weston Nurseries in Hopkinton, MA.

R.ponticum 'Variegatum' A popular variety with long dark green leaves enhanced by a white margin. It is an upright vigorous grower, hardy to -15^F, and has reddish-purple flowers in spring. It roots well and grows easily.

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'President Roosevelt' A variety with a golden yellow variegation in the middle of the green leaf and a picotee flower of a red edge fading to a white center. This plant was discovered in Holland in the 1920's and is not very popular. It has limited hardiness and a propensity towards a poor root system. It has acquired the derogatory nickname of "Tumbling Teddy" due to its weak stems and their tendency to break off easily from the root ball.

'Yaku Princess Sport' A new variety which offers excellent possibilities. It has a distinct gold center in the leaf, contrasting well with the dark green margin. It has light pink flowers and appears to be stable.

Here is a link that might be useful: Variegated Rhododendrons and Azaleas By Dr. Philip Waldman

I grow R.ponticum 'Variegatum' and in my climate it's somewhat challenging plant in terms of finding the proper location cause white margined leaves are very succeptible to winter/early spring sunburns, but being placed in a more shady location it blooms very poorly.
Otherwise it's vigorous and easy to grow rhododendron with interesting long contorted white-margined leaves.
In 7b you most likely will not have my problems :-))