Pete, I think the OP wanted to train to teach such a course rather than take the course.
There are certificates than claim to train you to be a teacher trainer, but I have been a teacher trainer and met lots and never met someone who got into the profession that way.
The best ways of becoming a teacher trainer are:
- Take a teaching job at a school that trains teachers, e.g. offers the CELTA, and then volunteer to help by having trainees observe your lessons etc. and hence build up your role
- Apply for teacher training jobs even if you don't have the experience and qualifications they ask for.
- Take higher teaching qualifications such as the DELTA or an MA
- If you know what kind of qualification you want to be a trainer for and you don't have exactly that qualification, consider taking it even if you already have another qualification that is meant to be "higher"
- Take part in as much other training as you can, e.g. short courses like the CELTA YL extension or the LCCI FTEB or local workshops, and look at how the trainer does their stuff
- Get relevant experience by taking a TEFL management job where you give workshops and observe teachers, giving workshops for your local chapter of TESOL etc.
- Read and write for teacher training journals
- Suggest to your school that they start to offer teacher training. Via Lingua and Trinity are two organisations that give you a fair amount of help in setting up a new course.