I'm not sure on a good place to buy an EGT gauge and sensor. They are all fairly expensive because of the high temperatures they are exposed to and measure. Usually within a couple of inches of the turbo outlet is acceptable. The closer the better. Some people install them before the turbo in the exhaust manifold. I think on the 617's it is easiest to install a probe fitting into the exhaust pipe at the turbo outlet. I want to do this to my car, but haven't yet. Time, money, etc.