The 556 I wanted to get on GB went beyond what I'm willing to pay.  There's another one on GB that I'm interested in, but it is missing the rear sight.

What are my options?

Dude, I'm really not trying to be a smartass but try Google image search!

Mine has the little fold in iron sight in the rail? The one you are looking at might be the same, I just sight it in and fold it down and run a nice 1-6..

I see I wasn't really clear on what I was trying to get at. I was looking for the Sig style diopter rear sight. Regardless, a coworker heard me looking for a 556, and pointed me to a rifle that he knew was for sale that happened to be just what I was searching for. I called the guy today, exchanged FFL info, and sent him payment. Got it cheaper that what I was going to spend on one through a Gunbroker auction too.