Only in video game stores that sell older used stuff or in pawn shops.
all you need to do is buy a link cable that fits the color it will also fit the advance
No most Game boy games are non violent but it depends on what game you buy.
walmart
There are a few card reader you can buy but they are hard to get nowadays so I would say buy an arduino and do this: http://hackaday.com/2011/03/20/gameboy-rom-backups-using-an-arduino/
You can buy them at the market near my house
Replace the entire battery back or run it entirely off the AC adapter or buy a new Game Boy.
Ebay or VideoGameBoxProtectors.com
You can buy larger expansion battery packs for the Game Boy Advance that last much longer.
buy it in Malaysia i got one
The Lenmar Replacement Battery for the Nintendo Game Boy is a good battery.
If you want to buy a Game Boy Advance nowadays, you'll most likely have to look at Ebay.
you can buy them off eBay