A dollar or two depending on where you get it.
For touching or writing on the touch screen.
The DS comes with at least one stylus and you can buy additional ones at various stores.
You mean the DS stylus? Why would GameStop give out the DS stylus for free? Short answer probably not.
you can use your pen or your finger as a stylus.
The best way is to keep the stylus in the stylus slot and then keep the DS in a sealable DS case. That way, it will not slip out and it will always be where you left it. Other options include buying a bigger stylus that is easy to find.
You can play games on the DS using a stylus and a touch screen
no, the 3ds stylus is different build from the ds stylus so the ds stylus cannot fit into the 3ds.
nintendo.com
with your stylus
Yes
its the DS pen that you get when you buy your DS (the one that you tap stuff with)
Yes there are quite a few that the stylus is completely optional.