Been hunting all day for a regular Sentinel (The convertible) and no dice...
I've been trying every tip in the book that's already been posted too.
I keep joining different invite only sessions every now and then.
I've been scouring Rockford Hills, Del Pierro, and Richard's majestic for hours in both my XS as well as my Jackal (Because I heard they tend to spawn when you're driving one).
It's getting annoying. I wish I held onto the Kryst4l one that I had
Kill the Black Sentinel guy and take his car. You have a better chance to spawn one if your driving the XS