For a variety of logistical, political, psychological, and emotional reasons, a significant percentage of women who provide surrogate pregnancy services are working with gay couples, and they're preferring it. Apparently, gay couples are more sympathetic, they'll watch the surrogate's kids, they don't have the difficult underlying competitive fertility issues that can complicate MF couples using surrogates. The list goes on.
The NY Times has a very thorough overview of gay parents' increasing use of surrogacy instead of adoption to start families. Frequently, one partner's sperm is joined with a donor egg, and the embryo is carried to term by the surrogate, who receives around $20,000 (on top of all medical expenses). The resulting genetic link between parent and child can make it easier for gay men to navigate the varying state-by-state maze of parenting rights.
The article also mentions "the close-knit surrogacy community," but it doesn't really point to many organizations or groups beyond the surrogate agencies. [That said, Circle Surrogacy in Boston, Growing Generations in LA, and Center for Surrogate Parenting in Encino, are all mentioned as gay-friendly or gay-focused surrogacy agencies.]