Any sporting goods store, most pawn shops, Walmart or KMart if they still sell firearms in your state.
Size of the bore.
Yes
http://www.e-gunparts.com
Remington
100-8000 USD depending on EXACTLY what you have and its condition.
If you can find one, yes.
1200.00
100-1000 USD
You find a gunsmith.
== ==
None published. Try shotgun collectors.
Yes. The Mossberg 500A1 is a .12 gauge shotgun.