Most commonly grown and widely available white is 'Cunningham White', one of the the so-called 'iron clads rhododendron'.
You may want to check your local box-store (HD, Lowe, W-Mart, etc) for availability.

You are in luck. One of the best rhododendrons ever is "Boule de Neige". It is very hardy, grows to about 5' tall, has outstanding leaves and among the most perfect snow white flowers. It is one of the most perfect landscape plants 12 months of the year. Its bloom is long lasting and very prolific. It is heat and sun tolerant but even blooms in deep shade. It is one of the better shaped rhododendron plants. It is readily available at better nurseries.

Other good white rhododendrons for Zone 6 are:

Catawbiense Album which opens faint mauve and fades to white with green markings,
Cunningham White with purple & brown markings which is tolerant of many soil types,
County of York which has very large flowers with an olive blotch,
Ice Cube with a yellow blotch,
Yaku Angel which opens faint pink and fades to pure white on a small compact plant.

Two of the best rhododendron nurseries in the USA are:

Greer Gardens Nursery in Eugene, OR: http://www.greergardens.com/

Rare Find Nursery in Jackson, NJ: http://www.rarefindnursery.com/

There are many other good nurseries, and the best plants are ones that are field grown in your own area.

Here is a link that might be useful: Boule de Neige

Wow! Thanks so much for the info! This site is great. I am all set to go find just what I need now. I hope to be able to give such great info and advice after (many!) more years of experience in gardening.

any one have any cuttings of Boule de Neige in maryland? Nudge nudge wink wink

You can also have a look at Dora Amateis- a small white rhodo, the picture of which is in my recent post asking for an id.

Here is a link that might be useful: the post with pictures