There is a tutorial of the opening process here:
http://gameboyretro.co.uk/?page=tutorial&sub=opengb
Q: where I can get a proper screwdriver to open a Nintendo Gameboy?
A: http://www.goldenshop.com.hk/AI-trad/gba/sdriver.htm
http://www.gamechoiceclub.com/navigateProduct.asp?product_id=179
http://www.consoleplus.co.uk/product_info.php?pName=tri-wing-screwdriver-for-gbagbaspwii
http://www.play-asia.com/paOS-13-71-1e-70-1fe.html
http://www.mcminone.com/product.asp?catalog_name=MCMProducts&product_id=22-9036
http://www.flashlinker-shop.com/product_info.php?cPath=26_40_50_60_63&products_id=484
http://www.gamebits.dk/shop/skruetraekker-til-gba-651p.html
http://www.modcomshop.de/triwing-schraubendreher-p-2127.html
http://www.gameasylum.us/trscoptoforg.html
http://www.hardcore-gamer.net/tienda/product_info.php/cPath/48_25_163/products_id/1321
search google for triwing screwdriver ...
or, alternatively, you might try to file down one wing of a philips head screwdriver ...
I think it's better to use a nail as the starting point for a Nintendo screwdriver; the metal is soft enough to be filed manually, and a dremmel tool would do wonders /Cycler
How Do I open my Gameboy Color?
http://youtube.com/watch?v=lQikASmmpm8