380 auto. Many good brands of ammo out there.
Try want ads, gun shows, gun shops.
Russian made
100-1000 depending on exactly what you want/have.
2mm in case length.
Blue Book of Gun Values
You don't define what "better" means to you. Price? Size?
$75-$125, depending on model and condition.
Best left to a gunsmith
25-75 USD
Near the firing pin
The .380 automatic cannot shoot 9mm Parabellum ammo.