http://www1.faa.gov/avr/afs/afs200/afs220/part65.htm
Most of the original founder like "AFDTC" & "Sheffield" are well known in the business. On the other hand if they are not certified you won't be able to take the exam.
Be careful with the Distance Learning, as the question with the FAA is how to prove your tuition hours without attending the course. And I would think carefully before taking the option, try to speak to the FAA to confirm beforehand.
Good luck
