We started by generating a list of as many different reputable donor agencies as we could find. We then requested passwords for each agency's donor database. When we found a donor we really liked, we went with the agency that represented her. Had we found our perfect donor at one of then other agencies on our list, I would have felt equally comfortable using them instead. The agency we ultimately went with was Tiny Treasures.
FYI, I found the agency reviews on this site to be incredibly helpful:
http://www.greateggs.com/egg-donor-agency-reviews.html