In principle there is no difference doing an instructor's rating either full time or part time and every flight school with the appropriate teaching capacity should be able to do it.
The length of the course will ultimately depend on how much time you can spare for the course, as well as the availability of the school's resources in terms of instructors and aircraft.
I did mine part-time due to outside commitments and it took me 10 months to complete the rating. Another guy I know in a similar situation took 14 months. How long's a piece of string ? It's entirely up to you. Good luck.