Here is a site with a list of track day organizers. I don't think there's anyone who does them everywhere, so you have to find one that operates on the tracks that are in your area. Every organization has its own style, so you might look for reviews from riders who have ridden with the groups you are interested in.
I've ridden a few times with
sportbiketracktime.com. They do a pretty decent job. I've heard that some of these organizations try to crowd too many bikes onto the track at once - that's something to watch out for. STT limits the numbers, and divides us into three groups, Novice, Intermediate, and Advanced, and each gets the track for 20 minutes out of every hour. Novice is anyone who hasn't been before, and anyone who chooses to stay there - it has a lot of strict rules, instructions for what you are supposed to do in every session, and a classroom session in between track sessions where they teach you stuff. Intermediate is for anyone who has been on the track before and chooses to go there - it's the carnage class, lots fewer rules, more aggression. Advanced is invitation only, it's mostly guys with racing licenses, and guys who were invited up from Intermediate due to their mad skillz. I have always stayed in Novice.
Whoever you ride with, it's probably best to go for a weekday session if you can. The weekend sessions are way more crowded.