We were in Rome early Sept in 2014 and were there 5 nights. We were staying very close to the Colosseum (literally less than a 3 min walk) and on day three, I went over as soon as they opened to see if they had any slots open for the underground tour. I was prepared to not do it, and could have taken any over the next few days. We were able to get 3 spots for a tour just after lunch, and they also had some spaces later in the afternoon.
So if you can't prebook, and you can get over first thing and maybe have a few days in the city, you could luck into one. We wre lucky we were nearby, because we just went back to our room, had breaky, back over to do the Palatine hill, grabbed a quick lunch then it was time for our tour. If your heart is set on it tho, and you can't get one with coopculture, then you may want to look into one of the other tour companies. I think walks of Italy also did a nighttime one certain days of the week - that sounds cool - maybe they don't offer them now.