The Delta Society.
I have two pit-mixes and recently took a training course through the Delta Society (nationwide organization originally formed in Portland, OR) which registers therapy dogs. I plan on taking the evaluation with my female dog next time it is offered in my area. You can find the Delta Society online at http://www.deltasociety.org/Page.aspx?pid=183.
I was so very impressed by the professionalism and knowledge of the two women who taught the course and the two handlers who brought their dogs in to show us exactly what we will need to accomplish to pass the evaluation.