The equivalent of the black four-in-hand worn by males, the black Service neck tab (alternate spellings and designations include “necktab” and “tab tie”) is authorized for wear by females with a variety of uniforms in the Army, Navy, Coast Guard and Coast Guard Auxiliary, USPHS Commissioned Corps, and the U.S. Merchant Marine. (Coast Guard Uniform Regulations, COMDTINST M1020.6J, authorizes women to wear a pre-tied black bow tie in place of the black tab tie.)
The Marine Corps also has a black Service necktab that you can purchase here.
When wearing the necktab, its outer edges should be as parallel as possible to the collar’s outer edges, with an equal amount of the necktab visible on each side of the collar. Please note that this necktab is not the same as the dress necktab, which regulations variously describe as “crescent-shaped” or a “quarter moon.”