You can purchase any brand you wish as long at it is the correct size.
Buy a manual.
There are a wide variety of places where one can purchase a Ford Mercury Villager. The easiest place to buy a Ford Mercury Villager is by visiting a local Ford dealership.
walmart
Do a Google search for it or buy a repair manual.
Autozone and most all auto parts stores.
You'll either need to get that from a dealer or from a wrecking yard.
Any place that sells batteries can tell you the correct size to buy.
You can't. Buy a new on eBay. They send reprograming instructions with it.
You need to buy an adapter harness. Do not cut into your wiring harness.
This is as complex procedure. I suggest you buy a repair manual if it is not obvious to you, and you don't have experience.
I suggest you buy a repair manual. This is a complex operation that would take many pages to describe.
A battery terminal or a battery CABLE terminal. If the terminal is gone or destroyed on the battery, you need a new battery. If the cable terminal is terminal, you can buy an after market replacement or a complete new cable with terminal at most any auto parts store.