One place to begin is Art Net, although finding works by Healy is quite difficult. A good trick when attempting to buy or sell art from a particular artist is to go to those galleries in the largest community from where the artist lives or lived. That is where you will usually find interest in the artist's work. In the case of Sean Healy, this would be Portland, Oregon. To find you a dealer, run a search using pertinent words, such as "Portland, OR art galleries Sean Healy." The following is what came up, which includes one Portland gallery and one in Houston:
Elizabeth Leach Gallery (Elizabethleach.com)
Moody Gallery (Moodygallery.com)
Since both galleries have held fairly recent exhibits of Healy's work, they are more likely than most to currently have work to offer for sale, or to be more likely seeking work for their collections.