My husband looked into this recently and he started by ringing around the various Driving schools. ISM are one of the biggest so maybe try there. You do have to do a course, some places will pay for it but you are contracted to stay with them for a fixed period after that and other companies have you pay yourself for the course. There are regulations coming in soon where all driving instructors must have a creditation(sorry can't remember who it is actually from) and you will need that to work for any company.
Good luck