I don't carry an extra stock battery. Just use the 2100 extended and carry the stock one. I'm about to attempt the stock battery cover mod so the extended works with it. Hopefully I don't screw it up lol
I believe the 2100mah is still on sale for $25 through VZW. May not last as long as the 3800mah but you can always carry around the stock battery as well...
Edit: Just checked VZW website and it is still $25