Among the best places to see wild rhododendrons in their natural habitat is Nepal. The plant here is known as Gurans, and is the national flower of Nepal.
There are over 1,000 species of Rhododendrons, half of these species being located in the Himalayas. The plant grows well at high altitude so you're likely to see them while trekking. Some present as flowering shrubs, others as trees up to 100 feet tall. Forests of these trees in bloom are an especially impressive sight.
Sadly the rhododendron forests are disappearing. Firewood is needed by the Sherpa people, and as the population and tourism grow they have turned to cutting down rhododendron trees to survive.