My troy built tractor 313AN779G766 WILL NOT START .The starter spins but won't engage.
Wher can I buy parts for the starter or a new starter? Jim

Your starter may be fine but the bedix gear is bad. The bendix is a sleeve on the starter, a gear candy canes down the sleeve and slams into the flywheel to start the engine. The teeth on the gear are the weak point and the first to go bad. A few times I've found a sleeve that sticks and the gear can't slide. Most tractor supply shops should have this part. You can remove the starter and inspect the gear to confirm the problem.

That is where I would suspect they will find wrong, either they need to lubricate the bendix, or replace. This is a simple repair for the diy'er.