possible causes of car shutting off while driving and then not turning over until starter is jumped?

This sounds like a bad electrical condition. It could be a severely depleted battery (the lead plates corrode and break down and cause shorts inside the battery) or alternator is failing or just bad grounds in the underhood wiring.

battery is new; alternator voltage checks fine and others have said bad grounds in the wiring; what's the best way to find bad grounds when no codes come up?

Just start tracing them out by site and take them apart, clean both mating surfaces up with a fine grit sandpaper or emory cloth and put them back together. Most grounds that fail are usually in the engine compartment and are usually a black wire that bolts to the engine and or body with a self tapping screw or nut & bolt. You can also find the type that are made of flat braided wire with either an eyelet that gets bolted down, or a clip with small teeth on the ends that slip over a sheet metal part. The last type tend to fail a lot. I once owned a chrysler mini van that had so many electrical gremlins in it, that I took a #10 green coated wire, and ran it around the engine to different mounting points, then to several different mounting point on the body in the engine compartment and finally to the battery ground and never had another electrical problem again.

You can also google wiring diagrams for your car and find just about every ground wire on the vehicle on those, or at the very least, buy a Haynes Manual for your car if you do your own repairs. One last thing, if you only tested the alternator voltage coming out of it with a meter, it does not mean it is good, sometimes as they heat up they make and break connection causing voltage spikes or drops & I have even had more than one of them test as good at auto parts stores, when in fact they were bad, after heating up.