I've trained with a bokken for many years, mostly though instruction by students in various arts and David Lowry's text. I'm at a point in my life where I would like to formalize my knowledge and receive more formal and direct instruction in a Japanese sword art, specifically Iaijutsu. However, finding an instructor/school is proving to be a challenge. So my question is this: is there a central body/organization in the US that focuses, at least in part, on promotion of Iaijutsu in the US?
I don't think there are any major umbrella iaijutsu organizations. The All United States Kendo Federation also supports iaido and you'll sometimes find (for instance) Muso Jikiden Eishin Ryu or Muso Shinden Ryu instructors among the local kendo sensei. I used to study kendo and iaido in the Pacific Northwest and I thought the quality of the instruction available was very good; if I was in your position I'd start with the AUSKF dojo list and see if there was an iaido school in my area.
That said, depending on where you are, there may also be excellent independent teachers or teachers affiliated directly with Japanese schools or organizations.