I cannot recommend an arborist, but do check that any arborist you choose is certified. The best time to prune rhodies is after bloom, and before next year's blooms start to form. So the time is soon. The following website has information on the best time to prune and how to best care for rhododendrons: http://extension.oregonstate.edu/gardening/node/775
Ask friends and neighbors who have rhodies for recommendations, if you can. Check the internet for local arborists, and some sites may have ratings.