S&L is correct.
It is not the instruction in itself but the payment for this which makes it imperative that the instructor has the appropriate CAA/JAR tickets when instructing for the FAA/IR.
The FARs do allow a non FAA instructor to teach but only outside the US. You will however need to be signed off to be 'checkride ready' and this will have to be done by an FAA instructor.
Mind you having been through the system myself I would think you would be less well prepared for the examination if you would not at least have had some exposure to someone who knows the system from 'within' as the examination with the oral and checkride is a bit different from the CAA/JAR format.
FD