Many of the doctors in this article signed on at the urging of a colleague. Another way is to keep your eyes open for magazine articles. Similarly, Operation Smile was written up in Good Housekeeping. And magazines commonly include public service ads for these organizations. As you are narrowing your choices, be aware that each group has different requirements for length of stay as well as pay.
Some groups offer a stipend.
Others will do no more than pay for your plane trip, while still others want you find an organization to subsidize your work. Some physicians say that it's important, too, to examine the politics of the organization with which you're considering an affiliation. Some physicians want a group that is active in speaking out against abuses in that country. Others may not. If you are chosen for an assignment, don't expect to get much notice.