Alrighty welcome to the forums.
Sadly the bios of the Playstation 2 is copywrited so downloading it would in most cases be ileagle hence we cannot tell you where to download it.
However it were more than likely tell you the name of the file you are looking for on the site you downloaded your emulator or if someone here knows the file name they may be kind enough to share it (I don't know it) Once you have the file name try Google-ing it and you should find what your after.
Remember though PS2 emulation is still in it's early stages so don't be surprised if it doesn't work as well as you expected.
In the words of ZSnes "Remember that this is a public beta so don't expect this to run on your machine."