Head to your local Japanese bookstore. Unlike most American bookstores, Japanese bookstores offer a range of other products, including toys, art supplies and comics (or manga). You'll usually find the futanari and other ero-manga mixed right in with the other graphic novels. Ask if there is a special section for English-translated versions.
Special order futanari at your favorite comic book store. While many comic book stores do not stock adult titles regularly, they may still have relationships with these publishers. It's helpful if you know the name of the publishing company and the item number for the manga you want to order.
Take advantage of the staff at the comic book store. You'll find most employees have vast knowledge of graphic novels and manga in particular. Remember, it's less embarrassing if you ask for futanari instead of "dickgirls."
Attend conventions. Besides being the world's largest comic book convention, the annual Comic-Con held in San Diego has whole areas dedicated to ero-manga, as does its sister convention, Wondercon (see Resources below). Smaller comic book and anime conventions take place throughout the year.
Keep your eye out for doujinshi, which is self-titled manga, usually from amateurs. Occasionally, you'll find a gem from a respected manga artist who is merely dabbling in futanari.
Stop by a futanari forum and ask. It sounds too easy, but the reason most people join a forum in the first place is to share their interests. In the larger communities, you'll find hordes of people posting images and links to manga sites.
Check out blogs dedicated to Manga, Japanese pop culture, **** and, of course, Futanari. Let the bloggers do all the research so that you can just drop by your favorite sites for fresh doses of futanari manga.
Stop by FutaHentai to view full versions of popular futanari mangas (see Resources below). You'll find a large selection of Japanese manga, but also a few scattered English-translated books. This is one of the few places that you can get free, full versions of the books online.