Exotic Yoga Retreats lists venues around the world in out of the way places, as the name suggests. Search by destination or date.
Kripalu Center for Yoga & Health in Massachusetts has yoga classes, wellness, creativity, Ayurveda, meditation, and more.
Yogaville, in the countryside of central Virginia, has many programs year-round. Recommended by personal friends, and although I’ve never done a retreat here, I have visited to check it out.
Yogaworks lists retreats and classes by location and pricing.