You need one that is accredited in your state. Check their sites and look over or download the materials. You usually don't have to pay for it until you actually use their service. If they all seem the same, it really doesn't matter which one, as long as it works, unless you think you'll be doing this often.
I did this, once, and the actual course was amazing. The way it was organized, you couldn't just flip throught the materials to get answers for the tests. You had to read it, start to finish, to know where to find the answers. The idea was obviously to make sure you really do read it, instead of skimming for answers, and it worked.