Here is a link for a relatively cheap OS that would be compatible with your Mac. Its a little old but will get you back on track. This is OS X 10.3
This is a newer version of OS X (10.4 Tiger). Its more expensive, but will get more updates than Panther...
This is the highest OS that the PowerBook can run, and is more expensive. The biggest advantage of this OS (Leopard) is that it will get the most updates available for PowerPc Macs...
Any of these will get your Mac back to normal operation. Just insert the CD/DVD and follow the on screen instructions!