This is where I got my game bit drivers: http://www.newelectronx.com/

You may want to just buy a tri-wing screwdriver while you are ordering if you ever plan to clean portable Nintendo games. You can buy the "screwdriver" handle for the bit if you don't already have a bit driver.

The reason it happens is just plain old dirt. Dust from the environment when not stored properly, inserting the game into a console with dirty contacts, blowing on the contacts, etc. all lead to the problem over time. Keep all of your games and systems clean, and always clean any new games before playing and you can cut way down on these problems.