Assuming your using a standard 80mm case fan like
this one....
There are a couple of options I see... The easiest and safest, I think, would to find an old AC->DC transformer like (the kind used to power cell phones, PDAs, switches, routers, etc.)
this. Make sure it outputs 12 volts, cut the end off the fan and transformer wire, and wire them up.
If that's not possible for what ever reason, you could use the power from USB, FireWire, PCMCIA, the hard drive, the optical drive, CPU/GPU fan, LCD, or some random lead on the motherboard.
USB would be the second choice, I think, safer and easier than any other. Unfortunately most of the things I listed above only output +5V. FireWire is variable from +8V to +30V but I have no idea how to control it. To use USB just find a female USB cable, splice the wires, hook up the USB's +5V wire to the fan, and plug the USB in. The down side is that your fan will spin slower than spec. (although it should be quieter) You might be able to use a transistor or voltage amplifier to bring it from +5V to +12V but I can't help you there.
Amperage might be a problem if your fan draws too much current. Check that to make sure.
Using your PowerBook's powerbrick thingy might also be an option.
Pinouts for some of your options:
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=pinconhdd_ata50_int
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=pinconhdd_ata44_int
http://pinouts.ru/data/ieee1394_pinout.shtml
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=pinconbus_pcmcia
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=pinconbus_usb
