Are you talking about this kind of Sentinel?
There's no specific point to find it. You generally find the normal Sentinel on dense (Downtown) or rich areas. Grab one for you and take advantage of the "replica glitch" (when a lot of traffic cars are the same model as the one you drive). You may find a tuned one in the traffic.
Warning that this parts aren't very frequently, so try to drive around areas where traffic jams are common, to increase your changes. Hope this helps.