How big is your hard drive? Although you can replace it yourself, it could void your warranty.
Anyway, at NewEgg.com, they don't have the 100 GB hard drive in stock, but there are plenty of 80 GB, 5400 RPM drives. You should definitely consider upgrading to a 5400 RPM if you want to replace your drive. An 80 GB, 5400 RPM will run you about $160.
But if you have a 1.25 GHz PowerBook, that means you have an 80 GB hard drive. If you're running out, you should just get an external case and a desktop hard drive for it. 3.5" Firewire cases run about $40 on NewEgg, and you can get an 80 GB drive for $60 or a 160 GB for $90.